Line two 10x2-inch cake pan with parchment, and spray them with nonstick cooking spray. (9-inch pans can be substituted but will take a little longer to cook through.) Preheat the oven to 350 F.
Add the butter and both sugars to the bowl of a large stand mixer fitted with a paddle attachment. Beat them together on medium-high speed, until light and fluffy, about 5 minutes. Add the eggs one at a time, beating well after each addition. Add the vanilla extract and mix it in.
In a separate bowl, whisk together the buttermilk, sour cream, and water (or coffee, if using). In a different bowl, sift together the flour, cocoa powder, baking soda, and salt. With the mixer running on low, add a quarter of the sifted flour mixture. When the flour streaks have almost all disappeared, add a third of the liquid to the mixing bowl. When that’s incorporated, continue to add the drys and wets in an alternating pattern, ending with the dry ingredients.
When the dry ingredients are nearly incorporated, stop the mixer. Scrape down the bottom and sides of the bowl with a rubber spatula, and finish mixing the cake by hand. Pour the batter into the prepared pans and smooth it into an even layer. Bake the cakes for 16-18 minutes, until a toothpick inserted into the center comes out with just a few moist crumbs attached. Cool the cakes in the pans for 10 minutes, then flip them out on a wire rack and let them cool completely. Repeat a second time so that you have 4 cake layers.
The cakes can be made several days in advance and kept, well-wrapped and refrigerated or frozen, until ready to use.
To Make the Hazelnut Frosting:
Preheat the oven to 350 F. Spread the hazelnuts out on a baking sheet and roast them in the oven for about 7-10 minutes, stirring several times during the process so they get evenly toasted. Stop when they're fragrant and golden brown.
Transfer the hot nuts to a food processor and process at full speed until they are a smooth paste, scraping down the sides of the processor when necessary. Let the hazelnut paste cool to room temperature.
Place the hazelnut paste, powdered sugar, butter, vanilla, and salt in the bowl of an electric mixer fitted with a paddle attachment. Mix on medium-low speed until creamy, scraping down the bowl with a rubber spatula as you work. Add 2 tablespoons of cream and beat on medium-high speed until the mixture is light and smooth. Add more cream if necessary, to get a light and smooth texture. The hazelnut frosting can be made in advance and kept in an airtight container in the refrigerator until you're ready to use it.
To Make the Vanilla Buttercream:
Combine the egg whites and the granulated sugar in the bowl of a large stand mixer, and whisk them together. Choose a small saucepan that lets you fit the base of the stand mixer snugly into the top of the saucepan—this is your makeshift hot water bath. (Alternately, you can use a different bowl or an actual bain marie and then transfer the mixture to a mixing bowl once it’s heated.) Add an inch of water to the bottom of the saucepan, and bring the water to a simmer.
Place the mixing bowl on top of the saucepan, making sure that the bottom isn’t in contact with the water, and heat the egg white mixture. Whisk frequently so that the egg whites don’t cook. Continue to heat the whites until they are hot to the touch, and when you rub a bit between your fingers, you don’t feel any grittiness from the sugar. Once the whites are hot, transfer the mixing bowl to your mixer and fit it with a whisk attachment.
Beat the whites on medium-high speed until they are a shiny, stiff, voluminous meringue, and are no longer warm to the touch—feel the outside of the bowl, and make sure that it is around room temperature. Depending on your mixer and the temperature of your environment, this may take 15-20 minutes, or more. Reduce the speed to medium-low and add the softened but cool butter in small chunks, a tablespoon at a time, making sure to wait in between additions. It may separate or look a little gloopy at this point—fear not. Once all of the butter is added, increase the speed again and whip until it comes together and is light and fluffy. If, after 5 minutes, it hasn’t come together, refrigerate the mixing bowl for 5-7 minutes, to cool the mixture down, and whip it again until it has thickened and is smooth and silky. Add the vanilla extract and salt.
The buttercream can be made in advance and kept at room temperature if you’re going to use it the same day, or refrigerated. If it has been chilled, let it sit at room temperature until completely soft, then re-whip it to get the smooth texture back before you use it. Before use, divide it and tint it to whatever colors you desire.
To Make the Banana Ganache:
Place 1/2 cup of heavy cream in a small saucepan over medium heat. Heat it until it simmers and small bubbles form all along the edges of the pan, but do not allow it to come to a full boil.
While the cream is heating, prepare the white chocolate and banana mixture. If you have a food processor, place the half banana in the food processor and mix until the banana has liquefied.
Add the white chocolate chips to the banana and process until the white chocolate is in small pieces.
The mixture will clump together, so stop the processor several times to scrape it down. Once the cream is simmering, with the processor on, carefully pour the hot cream in a small stream down into the bowl of the processor and let it run until the white chocolate has melted and the ganache is smooth and free of lumps.
If you do not have a food processor, this can be done in a mixer, or you can mash the banana by hand and add very finely chopped white chocolate to it, stirring until well combined. Pour the hot cream over the white chocolate-banana mixture and whisk it until it is smooth. This method produces a ganache that tastes the same, but there might be small pieces of banana that are not incorporated.
Once everything is smooth, scrape the ganache into a bowl and whisk in 1 tsp of banana extract. Taste the mixture, and add more extract if desired, to get a strong banana flavor. At this point, you can pause, press a layer of cling wrap on top of the ganache, and leave it on the counter for several days before proceeding.
When you're ready to assemble the cake, whisk together the gelatin and cold water in a small bowl. Let it sit for 10 minutes so that the gelatin absorbs the water. Microwave the bowl for 15-20 seconds, until the gelatin is liquefied. Microwave the banana ganache in 20-second increments, whisking after every 20 seconds, until it has softened and is the texture of peanut butter.
Beat the remaining 1 cup of heavy cream in a mixer until it holds medium peaks—it should be pillowy like shaving cream, not be curdled or crumbly. Whisk the melted gelatin into the softened ganache, then fold in the whipped cream in 2-3 batches, gently stirring so as not to deflate the cream. The banana mixture will start to set after a time, thanks to the gelatin, so it's important to start assembling the cake right after you add the gelatin and whipped cream.
If necessary, trim the tops of the cakes so that they're level. Place some vanilla frosting in a piping bag fitted with a coupler or 1/2-inch round tip. Place one cake round on a 10-inch cardboard cake circle, and pipe a circle of frosting around the outer edge of the cake. Spread half of the banana ganache on the cake, smoothing it into an even layer inside the buttercream circle.
Place a second cake round on top, and pipe a frosting ring around the cake layer. Scrape the hazelnut frosting onto the cake, and spread it into an even layer. Repeat this process with a third cake layer, topping this one with the remaining banana ganache. Finally, place the last cake round on top, press down gently, and make sure the sides of the cake are straight. Spread a very thin layer of frosting on the sides and top of the cake—just enough to trap any stray crumbs. Refrigerate the cake for an hour, to set the fillings and firm up the buttercream.
After an hour, decorate the cake with the remaining frosting in the pattern of your choice. The cake can be stored in the refrigerator for several days, but for the best taste and texture, allow it to come to room temperature before serving.
You can also bake the cake in 9–inch cake pans, and increase the baking time a bit.