Gelatin Under The Sea Cake

This fun illusion cake features two layers of delicious cake, and an almost transparent gelatin (jello) layer, perfectly resembling the ocean floor!

Course Dessert

Cuisine American

Keyword cake, gelatin, jello, penguin

Prep Time 3

Cook Time 3

Cooling/Setting Time (Inactive) 12

Total Time 18

Servings 12 servings

Ingredients

Ganache

3 oz heavy cream
8 oz white chocolate bar

Jello

65 g gelatin powder (9 packets of Knox brand)
7½ cups water, cool
2½ cups granulated sugar
2 tbsp lemon juice
22 g Berry Blue Jello powder (about a quarter of an 85 oz package)

Instructions

Step One: Prepare your bottom cake layer(s).
First, decide if you are going to place your jello layer in between the two layers of cake (we'll refer to this as Design/Photo A), or put the jello on top of the cake layers (we'll refer to this as Design/Photo B). (Note, my Design B is shorter than yours will be since I was working on the same cake to demonstrate both methods.)
Use your cake wire to level your bottom cake layer, and secure it to the cake drum with a bit of buttercream.
If you are putting both cake layers underneath the jello (Design B) apply a layer of buttercream to the bottom cake layer, then add the top cake layer onto it (as if you are stacking a normal layer cake.)
If you plan to sandwich the jello layer in between the cakes (Design A) set your top cake layer aside until the very end of this tutorial — you will not use the top layer of cake until Step 8.
At this point, when we refer to "the cake" we are referring to the one or two layers on the cake board, depending on which design you chose. To reiterate, if you plan to sandwich the jello layer in between the cake (Design A) you will not use the top layer of cake until Step 8.
Then, coat the top and sides of the cake on the cake board with buttercream, scraping down the sides to smooth them as you would when decorating any normal cake. Though this is technically the crumb coat, you'll want to create a thicker layer, with no cake showing through. It doesn't have to be perfectly smooth.
Chill the cake in the refrigerator while you prepare your next step.
Step Two: Make and cover your cake with ganache.
If you're stacking the jello on top of both cake layers (Design B) you'll need to make one and a half of the ganache recipe (or double it and have some leftover.)
Chop your chocolate bar into small pieces. Place it into a microwave-safe dish or bowl — the shallower the better. Heat the chocolate on 50% power (or the defrost setting) in increments of 15 seconds, until some of the chocolate just starts to melt (but don't let it all melt.)
Then, heat the heavy cream in the microwave in a cup or bowl until it starts to scald, or bubble. Then, pour it over the chocolate and let it sit, untouched, for a minute.
Slowly start to stir the mixture from the center, with small circles increasing in size until the heavy cream and chocolate are fully incorporated. If any chocolate remains unmelted after the ganache has fully come together, heat the mixture in the microwave just until it melts.
Let it cool to room temperature before using. Then, pour or spread it over your cake to completely cover the buttercream — on the top and sides of the cake. Make sure to spread it out with a spatula if it's not spreading on its own. Ganache stands up better than buttercream to the wet gelatin we'll pour on later, so it's important not to skip this step! The ganache helps seal the cake so no gelatin leaks into it.
Chill the cake again.
Step Three: Prepare gelatin and cool.
In a large saucepan off of the heat, combine the gelatin powder, cool water, sugar, and lemon juice. Mix it well, and then let it sit (off heat) for five to ten minutes, or until the gelatin has fully dissolved. This is called "blooming" gelatin, which helps strengthen it.
Then, add the Jello powder, mix, and turn the heat on to medium-high. Let the mixture cook for a while, stirring occasionally. However, do not let it boil. Once the jello begins to steam, remove it from the heat and pour it into a bowl. (I only like to use a small amount of the Berry Blue Jello powder, because, otherwise, the flavor and blue color are too potent.)
Chill the bowl of jello in the refrigerator until it has cooled to room temperature — this took me about an hour. Meanwhile, work on the next step.
Step Four: Mold under the sea creatures from modeling chocolate.
Since it's mostly sugar-based (and water-soluble at that) you cannot use fondant to make figures for the inside of your gelatin, otherwise, they will disintegrate. However, modeling chocolate (like ganache) holds up much better.
Pinch off a small amount of modeling chocolate, and knead it on a silicone baking mat until it becomes pliable and "clay"-like. Apply a small amount of powdered food coloring pigment before kneading to color the chocolate based on your design.
Once the coloring pigment is mixed in, use the fondant tools to create your characters and other under the sea decor. I won't go too deep into modeling and molding, but I've found it helps to think of animals and plants as simple shapes, first, and then form or add details.
For example, to make the fish, I first rolled small balls of chocolate into cylinders. Then, I pinched one end into a rounded point to create the face and flatted the other into the tailfin. I used the sharp fondant tool to cut a notch between the fins. If you're struggling to create something, try searching for modeling videos online — even if the video is for clay, the same techniques will apply.
Cover your unused modeling chocolate with plastic when you're not using it so it doesn't dry out. Let finished pieces harden, uncovered.
I traced my cake pan onto parchment paper so that I could compare the sizes of the characters with the cake, and make sure everything fit nicely. There is nothing more frustrating than making a bunch of cute elements, only to pull out your cake and find that they don't fit properly.
Step Five: Mold your fondant figures (optional) for the top of the cake.
Since fondant (in my area) is much cheaper than modeling chocolate, I chose to use fondant for the characters that will go on the top of my cake, away from the gelatin.
However, you can choose to continue with the modeling chocolate, too.
Similarly as with the under the sea creatures, mold and form the figures you wish to go on top of your finished cake. Set them aside to harden and dry.
Step Six: Prep your cake for gelatin.
Carefully cut a piece of acetate, slightly longer than the circumference of your cake (so that it can wrap around it fully.) Pull your chilled cake out from the refrigerator, and carefully wrap the cake with the acetate. Use your hands to smooth the edges down, making a good, tight seal against the ganache. You can even pour some leftover ganache in between the acetate and the cake if you have leftovers and notice gaps between the two surfaces.
Then, expand your cake ring and place it over the acetate, sliding it down to firmly rest on your cake board. Tighten the cake ring so it's very snug against the cake.
For added protection, apply tape or use a finger to smooth some ganache against the bottom of the cake ring and your cake board. This ensures that no jello can leak out. I had a few spots where I noticed the jello leaked out and was caught by the tape, so I was glad I used it! Pop the cake back in the refrigerator to set the new ganache, if you used it. If the ganache isn't chilled when you add your gelatin, it could run together and make a mess!
Step Seven: Pour your first gelatin layer.
At this point, place onto the cake your modeling chocolate figures that should rest on the ocean floor (like greenery, coral, shells, etc.). If you want the fish or other items to appear as if they are "floating" or "swimming" leave them out of the cake for now.
Then, melt a bit of your cocoa butter in the microwave and use a food-safe paintbrush to apply the butter to any modeling chocolate on the ocean floor, as well as the items you've left to the side. The food coloring pigment has a tendency to leak into the gelatin a little as it sets, so this cocoa butter layer prevents that from happening.
Then, with a ladle or measuring cup, carefully spoon onto your chilled cake a small amount of the room temperature jello. Pay attention to pouring the jello around the edges, too. However, only add about a half-inch or so of jello. Then, let it set in the refrigerator for 15 to 20 minutes. This helps to form a stable barrier so that your jello won't majorly leak out when you apply it all.
Step Eight: Pour another gelatin layer.
Next, add a fish or item that you want to "hover" over the ocean floor. Then, carefully spoon in another half-inch or so of jello, and let it set again. Repeat this process, adding characters as the jello layers set, to give a more realistic appearance. (I only did this twice.)
Finally, when all of your under the sea creatures are in place, pour in the remaining jello. Let this set in the refrigerator at least overnight, or, depending upon the thickness of your layers, for up to 24 hours. My jello filled an 8-inch circular area about two inches deep and took about 12 hours to set fully.
This step is optional for Design A — to create a "pond" for the top layer of cake, I lined a cake pan with parchment paper, lightly oiled it, and then place a large mug (opening facing down) into the center. Then, I poured the small amount of gelatin I had left into the pan and let it set along with the cake.
Step Nine: Unmold the jello.
Once your jello layer looks and feels sufficiently set, remove it from the refrigerator. Peel away the tape from the ring, if you used it. Next, very carefully, expand the ring. Slide it up and over the acetate to remove it. Then, slowly peel away the acetate layer to reveal your jello layer!
However, if at any time you notice liquid jello beginning to leak out from the bottom (like after you remove the baking ring) place it back in the refrigerator to set for a few hours more. If it's only a small amount of leakage, place the ring back on and pop the cake into the freezer for 15 minutes, then remove the ring and place the cake in the refrigerator until it's fully set. However, you should never let jello freeze for a long period of time.
Step Ten: Add your decor and (optional) top layer.
Use the following steps for design A:
On a clean turntable, apply a smooth layer of buttercream to the top and sides of your inverted top cake layer. Then, chill until the buttercream sets. It's harder to work on top of the wiggly jello layer, so I like to add a thick crumb coat before placing the cake layer on the jello one.
Once the buttercream is set, carefully use an offset spatula to move the cake layer from the turntable to the jello layer — you'll want to set the unfrosted surface of the cake directly onto the jello for the most secure attachment.
Unmold your "pond" (if you made one) by carefully running a knife along the edges, and then lifting it out. Place it on your top cake layer.
Apply a small amount of buttercream around the edges of the pond (as a barrier) and on the inside of the pond. This is easier with a piping bag because the jello will slide around on the buttercream. The "dam" of buttercream, piped partially on the "pond" and partially on the butter-creamed cake will help hold the pond in place — think of it as "tape". I kept the edges kind of rough-looking so that they resembled natural snowbanks.
Use the following steps for design B:
To create an "island" on which my penguins sat, I spread some buttercream onto a piece of parchment paper. Then, I froze it and used a circular cookie cutter to remove a perfect circle. I then freehanded a sort of "island" design (on which I later set my ice cream cone trees) and froze that as well for easy application.
Use the following steps to finalize both designs, A or B:
Then, with an offset spatula, apply another layer of softer American buttercream to the outside edges of the cake layer(s). Do so carefully, avoiding overlapping the gelatin later too much.
Then, use your cake scraper to gently smooth out the buttercream. Do this as you would normally scrape a cake, but pay close attention not to tear or knick your jello layer. This is a similar method to making a fault-line cake.
Before serving, place your fondant or modeling chocolate figures on top of the cake, if you created them. (See storage notes below.)

Notes

Important:

This cake is best served as close as possible to finishing it. Though we take precautions to avoid it, the gelatin and buttercream tend not to play very well together, so the final touches of buttercream around and on the gelatin start to compromise its structure a little bit. Nothing crazy, but you may notice some moisture around the edges where the buttercream and jello meet after the cake has sat for a few hours.

If you have to make it in advance, which I don't recommend, keep this in mind: fondant and modeling chocolate tend to become sticky and gummy in the refrigerator, but because of the jello layer, this cake should be kept in the fridge. So I'd recommend adding the figures on top before serving. However, cake tastes best at room temperature, so let the cake warm up out of the fridge for an hour or so before serving.

This cake is a showstopper, but the center layer of delicate jello makes it a bit difficult to transport. I'd recommend placing any fragile fondant toppers or buttercream-covered ice cream cone trees on the cake after traveling if at all possible, to avoid them toppling over. After all, wiggly jello is what the fun treat is known for!