Preheat the oven to 350F. Line the bottom and sides of a 9-inch round, 3-inch deep springform pan with parchment paper.
Add the eggs and sugar to a large bowl. Use a handheld electric beater to beat until the mixture is light and fluffy, about 5 minutes. Sift in the flour, cornstarch, baking powder, and salt. Use a rubber spatula to fold the dry ingredients into the wet, being careful not to over-mix.
Pour the batter into the prepared pan. Bake until the cake is golden and puffed, about 20 to 30 minutes.
Cool the cake completely, about 30 minutes.
For the Sour Cream Mousse Layer:
Once the cake is cool, make the sour cream mousse layer.
Add the sour cream and sugar to a large bowl. Use a handheld electric beater to beat until the mixture is light and fluffy, about 7 to 10 minutes. Add the sweetened condensed milk and vanilla, and beat to combine. Set aside for now.
Add the gelatin and milk to a saucepan over medium heat, whisking to combine. Bring to a boil, and then add the butter and salt. Continue cooking until the butter is melted, whisking frequently. Remove from the heat.
Whisk 1 cup of the sour cream mixture into the gelatin mixture to cool the gelatin mixture a little. Now gradually drizzle the gelatin mixture into the sour cream mixture while whisking constantly. Continue whisking until it’s well-combined.
Pour the sour cream mixture onto the cooled sponge cake. Pick the cake up a half inch or so from the countertop and gently drop it to release any trapped air bubbles; repeat this a couple times. Transfer the cake to the fridge to chill completely and set, about 4 hours. You’re ready to move onto the next step when the sour cream mixture is chilled and jelled.
Once it’s jelled, remove the side of the springform pan and discard the parchment paper that was on the side. Place the cake on top of a wire rack set on a baking tray because we’ll be topping it with chocolate.
For the Chocolate Ganache:
Once the sour cream mixture has jelled, make the chocolate ganache.
Add the chopped chocolate to a bowl and pour in the hot cream. Let it sit for 2 minutes, and then gently stir to combine. The heat from the cream will be enough to melt the chocolate.
Let the chocolate mixture cool briefly (about 3 to 5 minutes) and then pour it on top of the jelled sour cream mixture, spreading it around with an offset spatula or an icing spatula. Let it drip over the sides as much as you want.
If you have extra chocolate ganache, chill it in the fridge, and then whip it. Transfer it to a piping bag fitted with a star tip and pipe decorations on top of the cake. If desired, decorate the top with edible luster dust and chocolate pearls.
Let the ganache set before slicing and serving (at least 1 hour in the fridge).
Notes
Recommendation: Read the entire recipe through before starting – this will make your workflow so much easier! Also, measure out all the ingredients before you begin.
To Get the Light and Airy Mousse-Like Texture: When you’re making the filling, be sure to beat the sour cream and sugar until light and fluffy! This takes about 7 to 10 minutes using a handheld electric mixer.
Make Sure to Let the Sour Cream Layer Chill and Set Completely: It must be fully chilled and set before moving on to the next step.
How to Tell When Sponge Cake is Done Baking: It’s done when you press the top and the cake springs back. If you press the top and the indentation remains, it needs a little longer.
If Your Pan Isn’t Deep Enough: I recommend using a 3-inch (or 4-inch if possible) cake pan because this cake is quite tall. If it doesn’t look like your pan is deep enough, you can easily remedy that. Once your cake is cooled, you can slice it in half horizontally. Place 1 half back into the pan and continue with the recipe; use the other half for something else. If you also have filling, you can crumble the extra cake into individual bowls and pour the extra filling on top.
When Making the Chocolate Ganache: Don’t stir aggressively because we don’t want to incorporate air into the chocolate. Spread the ganache onto the cake with an offset spatula or icing spatula so you can get the look you want. I like to let the chocolate drip over the edges just a little.
If You Have Leftover Ganache: Let it chill a little in the fridge, whip it, and pipe it onto the top decoratively. I also decorated the top with edible gold glitter!