Begin this recipe one day ahead of time. The mousse layer needs to freeze solid overnight. If you have Bake Even Cake Strips, then I suggest using them for the cake layers. They will sometimes eliminate the need to level the baked cakes (which was my result except for the red velvet layer). I used (and recommend) Baker’s Joy flour-based baking spray to grease the pans. This reduces the amount of crumbs on the outside of the cake and eliminates the need for lining the pans with parchment paper.
When all of the cake layers are baked and ready to be stacked, the flavors can be arranged to your liking; however, the red velvet layer is soft and should not be used as the bottom tier. Because the mousse layer is fluffy and light, it should always be on top.
You’ll need about 1 1/2 cups of sprinkles to cover the outside of the cake. Check the ice cream toppings aisle at the grocery store for large containers of sprinkles. If transporting this cake, make sure it is well chilled before departing, and consider doweling it in the center so the layers don’t shift while traveling.
Sprinkle gelatin evenly over the water in a medium saucepan. Let stand for 1 minute. Whisk in egg yolks and sugar together well (save the egg whites for the hummingbird cake layer). Stir in hot milk. Cook over medium-high heat, stirring constantly. Mixture will thicken after about 5 minutes. When done, it should be thick enough to coat the back of a spoon. Stir chopped chocolate into the cooked gelatin mixture and blend until chocolate has melted and even in color with no streaks of the yolk mixture. Allow to cool for 20 minutes, or until just warm.
Line and 8-inch round pan with plastic wrap so that the inside is completely covered and plastic wrap overhangs all sides of the pan. Whip cream until stiff peaks form. Gently fold whipped cream into the chocolate mixture until well blended. Pour mousse into the plastic wrap-lined pan and smooth the top. Cover and freeze until solid, overnight. When mousse layer is frozen solid, remove from pan and remove plastic wrap. Transfer to a plate and store in the freezer while the other layers are baked and assembled.
Hummingbird cake layer
Preheat oven to 350°F. Grease an 8-inch round cake pan.
Combine the first five (dry) ingredients. Add egg whites and oil, mix well. Add vanilla, pineapple, nuts and banana; stir to combine. Pour into the pan. Bake for 20-25 minutes. Let cool in pan 5 minutes. Turn out on a wire rack to cool completely.
Red velvet layer
Preheat oven to 350°F. Grease an 8-inch round cake pan.
In a large bowl, combine the dry ingredients (first 5 ingredients). In a separate bowl, gently beat together the oil, buttermilk, egg, food coloring, vinegar, and vanilla with an electric mixer. Slowly add in the dry ingredients to the wet and mix until thoroughly combined. Pour the batter into the pan and bake for 25-28 minutes. Remove from oven. Let cool in pan 5 minutes. Turn out on a wire rack to cool completely.
Vanilla sour cream cake
Preheat the oven to 350°F. Grease an 8-inch round cake pan.
Whisk together the sour cream, egg, sugar, vegetable oil, and vanilla extract.
Sift the salt, baking powder, baking soda and flour over the wet ingredients; mix until smooth. Pour the batter into the pan. Bake the cake for 20-25 minutes, until it is beginning to brown on top. Remove the cake from the oven, and after 10 minutes turn it out of the pan onto a rack to cool.
Chocolate cake layer
Preheat oven to 350°F. Grease an 8-inch round cake.
In a small bowl, whisk the flour, cocoa powder, baking soda, and salt. In a large bowl, combine the melted butter and brown sugar with a wooden spoon or rubber spatula. Add the eggs and vanilla; stir until well blended. Add the flour mixture all at once and stir just until all the flour is moistened. Pour the coffee or hot water over the batter; stir just until it’s incorporated and the batter is smooth. Scrape the batter into the prepared pan. Bake 30 to 40 minutes. Let cool in the pan on a rack for 10 minutes. Turn the cake out of the pan onto a rack to cool completely
Cream cheese frosting, toppings and assembly
In a large mixing bowl, beat the cream cheese, butter and vanilla together until light and smooth. Add the sugar and beat on low speed until incorporated. Increase the speed to high and mix until very light and fluffy.
Place the chocolate cake layer on a sturdy cake stand. Spread 1/3 cup of cream cheese frosting over the layer and top with the vanilla cake layer, spread with frosting as before and top with red velvet layer; cover with cream cheese frosting and top with hummingbird cake layer, spread with frosting and top with the frozen chocolate mousse layer. If the chocolate mousse layer is larger than the cake layers (they shrink a little in the pans, the mousse does not), trim the edges of the mousse layer so that it is flush with the sides of the other cake layers. Cover the entire cake, except for the very top, in a layer of cream cheese frosting. Place the frosted cake in a large baking pan and press handfuls of sprinkles onto the sides of the cake, allowing the excess sprinkles to fall back into the pan (do the best you can with this *wink). Please note! It’s important to cover the cake with sprinkles while the frosting is still tacky, so do this soon after covering the cake with frosting. Transfer leftover frosting to a piping bag fitted with a large star decorator tip. Pipe approximately 12 swirls of frosting around the top edge of the cake. Place a maraschino cherry on every other icing swirl. Sprinkle the swirls and cherries with pinches of rainbow nonpareils.
Store cake wrapped in plastic wrap in the refrigerator. Bring to room temperature before serving.