Step 1: Prepare the Chocolate Sponge Cake

1. Preheat your oven to 350°F (175°C). Grease and line two 9-inch round cake pans with parchment paper.
2. In a medium mixing bowl, sift together the flour, cocoa powder, baking powder, and salt. Set aside.
3. In a large bowl, beat the eggs and granulated sugar with an electric mixer on medium-high speed until pale and fluffy, about 5-7 minutes. The mixture should double in volume.
4. Add the melted butter, milk, and vanilla extract, mixing gently until combined.
5. Gradually fold in the dry ingredients, being careful not to overmix. The batter should be smooth and airy.
6. Divide the batter evenly between the prepared cake pans. Smooth the tops with a spatula.
7. Bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean. Let the cakes cool in the pans for 10 minutes, then turn them out onto a wire rack to cool completely.

Step 2: Make the Cherry Filling

1. In a medium saucepan, combine the cherries, sugar, and kirsch (or cherry juice) over medium heat.
2. Stir occasionally until the sugar dissolves and the cherries release their juices, about 5 minutes.
3. Add the cornstarch slurry (cornstarch mixed with water) to the cherries, stirring continuously until the mixture thickens, about 2 minutes.
4. Remove from heat and allow the cherry filling to cool completely before using in the cake.

Step 3: Prepare the Whipped Cream Frosting

1. In a large mixing bowl, combine the heavy cream, powdered sugar, and vanilla extract.
2. Whip the cream on high speed with an electric mixer until stiff peaks form. Be careful not to overbeat, as this can cause the cream to become grainy.
3. Chill the whipped cream in the refrigerator until ready to assemble the cake.

Step 4: Assemble the Cake

1. Once the cakes are completely cooled, use a serrated knife to slice each cake layer in half horizontally, creating four thin layers.
2. Place one layer of cake on a serving plate or cake stand. Brush the top lightly with extra kirsch or cherry juice for added moisture if desired.
3. Spread a thin layer of whipped cream over the cake layer, then spoon some of the cherry filling on top.
4. Place the second layer of cake on top and repeat the process, layering cake, whipped cream, and cherry filling until all layers are stacked.
5. Use the remaining whipped cream to cover the top and sides of the cake, smoothing it with a spatula.

Step 5: Decorate the Cake

1. Press chocolate shavings or curls onto the sides of the cake for a classic Black Forest look.
2. Garnish the top of the cake with fresh cherries and additional chocolate shavings if desired.
3. Refrigerate the cake for at least 1 hour to allow the flavors to meld before serving.