Prepare the chocolate sponge cake first. Preheat the oven to 350F/177C. Prepare two, 8-inch/20-cm cake rounds by lining the bottom with parchment paper; do not grease the sides. Place the eggs, sugar and vanilla and coffee extracts into a stand mixer bowl, then whisk on high speed for 7 to 9 minutes. The eggs need to be thick, pale in color and super fluffy. In a separate bowl, combine the dry ingredients: flour, baking powder and cocoa powder. Sift the dry ingredients into the egg mixture in small increments, folding gently but thoroughly after each addition.
Divide the batter evenly between the two baking pans. Bake in the preheated oven for 18 to 20 minutes, until the tops are set. They will spring back when tapped gently when done. Remove the cakes from the oven and place onto a cooling rack. Immediately, run a knife along the edge of the pan and cake to release the cake from the pan. Allow the layers to cool completely before removing them from the pans. Once the layers are cooled, remove the parchment paper and use a long serrated knife to split the layers in half.
Next, prepare the chocolate custard: In a medium saucepan, combine the milk and flour; whisk the flour in until no clumps remain. Cook the milk over medium-low heat for approximately 5 minutes, until the milk is steaming hot but not boiling. Meanwhile, in a separate mixing bowl, combine the egg yolks and sugar. Whisk vigorously for 2 to 3 minutes until the mixture is thick and pale and ribbons off of the whisk. In a small ramekin, combine the cornstarch and water to create a slurry. Add the slurry to the egg yolk mixture and whisk for 30 seconds.
Once the milk is hot, slowly temper the hot milk into the egg yolks. Add a little at a time and whisk well after each addition. Once the two mixtures have been combined, pour the custard base back into the saucepan. Cook the mixture over medium-low heat for 5 to 7 minutes, stirring constantly with a rubber spatula. Custard is ready when it holds its shape on the back of the spatula. Avoid cooking over high heat; this can cause the custard to separate. Remove the custard from heat and add the butter, chocolate chips and vanilla extract. Whisk the butter and chocolate into the hot custard until it's completely melted and smooth. Cover the custard and let it chill completely in the refrigerator.
Once the cake and the custard are done and cooled, prepare the cherries. Pit the cherries first, then quarter them and place into a medium bowl. Pour the cherry liqueur (or juice) over the cherries and let them stand.
Next, prepare the whipped cream for the mousse. Pour the chilled heavy cream into a mixer bowl and add the vanilla and coffee extract and confectioner's sugar. Whisk on medium to high speed for 3 to 4 minutes, until stiff peaks form. Reserve 1 to 1 1/2 cups of the whipped cream in a pastry bag for garnishing the top of the mousse cake later; keep it refrigerated. Meanwhile, combine the gelatin and 2 tablespoons water in a small ramekin. Heat the gelatin in the microwave for 30 to 40 seconds, until the gelatin is completely dissolved. Pour the HOT gelatin into the remaining whipped cream, with the mixer running on medium speed. Whisk for 1 minute.
Gently fold together the stabilized whipped cream and completely cooled chocolate custard. Do this in steps, adding 1/3 of the custard at a time and folding gently but thoroughly with a spatula. Watch my video recipe to see how it's done!
To assemble the cake, prepare an 8-inch springform and line the sides with an acetate cake collar or foil doubled over. Add the first chocolate cake layer into the bottom of the pan, then add approximately 1/3 of the cherries and cherry liqueur. Soak the sponge cake with extra liqueur if desired. Add 1/3 of the chocolate mousse and spread it evenly to the sides. Top with the second cake layer and repeat the process for the third layer, too. The fourth and final layer is optional; I opted to leave it off the top for my video recipe.
Refrigerate the mousse cake for a minimum of 6 hours or overnight for best results. Once the mousse is set, remove the springform and use a cake lifter to transfer the cake onto a stand. Once transferred, remove the acetate cake or foil. Garnish the top of the cake with the reserved whipped cream. I used Ateco tip #823. Add fresh cherries on top, if desired.
More tips: keep the cake in the cake collar and in the pan until ready to serve, or if transporting. The cake will stand in the refrigerator for 3 to 4 days and can also be stored in the freezer and enjoyed as a delicious ice cream cake!