Put a piece of parchment or a Silpat on the counter. Invert the cake onto it and peel off the parchment or Silpat from the bottom of the cake. Use the cake ring to stamp out 2 circles from the cake. These are your top 2 cake layers. The remaining cake 'scrap' will come together to make the bottom layer of the cake.

Clean the cake ring and place it in the center of a sheet pan lined with clean parchment or a Silpat. Use 1 strip of acetate to line the inside of the cake ring.

Put the cake scraps inside the ring and use the back of your hand to tamp the cake scraps together into a flat even layer.

Dunk a pastry brush in the milk and give the layer of cake a good, healthy bath of half of the milk.

Use the back of a spoon to spread half of the ganache in an even layer over the cake.

Sprinkle one-third of the hazelnut crunch evenly over the ganache. Use the back of your hand to anchor it in place.

Use the back of a spoon to spread half of the banana cream as evenly as possible over the crunch.

With your index finger, gently tuck the second strip of acetate between the cake ring and the top ¼ inch of the first strip of acetate, so that you have a clear ring of acetate 5 to 6 inches tall—high enough to support the height of the finished cake. Set a cake round on top of the banana cream and repeat the process for layer 1 (if 1 of your 2 cake rounds is jankier than the other, use it here in the middle and save the prettier one for the top).

Nestle the remaining cake round into the banana cream. Cover the top of the cake with all the hazelnut frosting. Give it volume and swirls, or do as we do and opt for a perfectly flat top. Garnish the frosting with the remaining clusters of hazelnut crunch.

Transfer the sheet pan to the freezer and freeze for a minimum of 12 hours to set the cake and filling. The cake will keep in the freezer for up to 2 weeks.

At least 3 hours before you are ready to serve the cake, pull the sheet pan out of the freezer and, using your fingers and thumbs, pop the cake out of the cake ring. Gently peel off the acetate and transfer the cake to a platter or cake stand. Let it defrost in the fridge for a minimum of 3 hours (wrapped well in plastic, the cake can be refrigerated for up to 5 days).

Slice the cake into wedges and serve.

Heat the oven to 325°F.

Combine the butter and sugar in the bowl of a stand mixer fitted with the paddle attachment and cream together on medium-high for 2 to 3 minutes. Scrape down the sides of the bowl, add the egg, and mix on medium-high again for 2 to 3 minutes. Scrape down the sides of the bowl once more.

Stream in the buttermilk, oil, and banana extract while the paddle swirls on low speed. Increase the mixer speed to medium-high and paddle for 5 to 6 minutes, until the mixture is practically white, twice the size of your original fluffy butter-and-sugar mixture, and completely homogenous. You're basically forcing too much liquid into an already fatty mixture that doesn't want to make room for it, so if it doesn't look right after 6 minutes, keep mixing. Stop the mixer and scrape down the sides of the bowl.

On very low speed, add the bananas and mix for 45 to 60 seconds to ensure all the bananas are broken apart.

Still on low speed, add the flour, baking powder, baking soda, and salt and mix for 45 to 60 seconds, just until your batter comes together and any remnants of dry ingredients have been incorporated. Scrape down the sides of the bowl.

Pam-spray a quarter sheet pan and line with parchment, or just line the pan with a Silpat. Using a spatula, spread the cake batter in an even layer in the pan. Give the bottom of your sheet pan a tap on the countertop to even out the layer (this cake batter is very forgiving). Bake for 25 to 30 minutes. The cake will rise and puff, doubling in size, but will remain slightly buttery and dense. At 25 minutes, gently poke the edge of the cake with your finger; the cake should bounce back slightly and the center should no longer be jiggly. Leave the cake in the oven for an extra 3 to 5 minutes if the cake doesn't pass these tests.

Take the cake out of the oven and cool on a wire rack or, in a pinch, in the fridge or freezer (don't worry, it's not cheating). The cooled cake can be stored in the fridge, wrapped in plastic wrap, for up to 5 days.

Bring the heavy cream to a boil in a small heavy-bottomed saucepan over medium-high heat.

Meanwhile, combine the melted gianduja, hazelnut paste, fudge sauce, and salt in a medium bowl.

Pour the cream into the bowl and let sit undisturbed for 1 minute. With a hand blender or a whisk, slowly mix the contents of the bowl until the mixture is glossy and silky-smooth. This will take 2 to 4 minutes, depending on your speed and strength. Use immediately, or store in an airtight container in the fridge for up to 2 weeks; do not freeze.

Put the butter in the bowl of a stand mixer fitted with the paddle attachment and paddle on medium-high speed until it is completely smooth. Scrape down the sides of the bowl with a spatula. This is a small amount of ingredients so use your granny mixer now or take on the task by hand in a medium bowl.

Add the hazelnut paste, confectioners' sugar, and salt and mix on high speed until the frosting is fluffy and has no lumps in it, 3 to 4 minutes. Scrape down the sides of the bowl and mix for 15 seconds, just to be sure everything is nice and smooth. Use immediately, or store in an airtight container in the fridge for up to 1 month. Bring to room temperature before using.