Cocoa Sponge Cake

1. Preheat the oven to 165°C (325°F) and line three 15 cm x 7 cm (6-inch) round cake pans with parchment paper.
2. In a large bowl, sift together the flour, dark cocoa powder, baking powder, baking soda, and salt.
3. In another large bowl or the bowl of a stand mixer, whisk together the eggs, granulated sugar, vegetable oil, and vanilla extract.
4. Add the buttermilk and coffee (or water) and mix until combined.
5. Gradually blend in the flour mixture until you have a smooth and dark liquid batter.
6. Divide the batter evenly among the prepared cake pans.
7. Bake the cakes in the middle of the oven for about 40 minutes without opening the oven door. (Test the cakes for doneness by inserting a toothpick into the center; it should come out clean.)
8. Allow the cakes to cool in their pans for a few minutes before running a knife around the edges and carefully removing them from the pans to cool completely upside down on a wire rack.

Black Cocoa Mascarpone Frosting

1. In a large bowl, whisk together the cold mascarpone, cold heavy cream, powdered sugar, vanilla, and cocoa powder until smooth and creamy.
2. Transfer the frosting to a large piping bag.

Assembly

1. Trim the tops of the cakes if needed to create even layers.
2. Assemble the cake in a deep cake pan or using an adjustable cake ring for straight layers.
3. Place the first layer of cake in the pan (lined with plastic wrap for easy removal) and spread a layer of mascarpone frosting, smoothing it with an offset spatula.
4. Cover with the second layer of cake and repeat until the final layer, then cover the cake with plastic wrap and refrigerate for 1 hour or 20 minutes in the freezer.

Mascarpone Cocoa Covering Frosting

1. In a large bowl, combine all ingredients: cold mascarpone, cold heavy cream, sifted powdered sugar, and black cocoa.
2. Using an electric mixer on low to medium speed, beat for about 1 minute until you achieve a thick and smooth frosting.

Smoothing and Decoration

1. Unmold the layer cake and place it on a cake board on a rotating cake stand.
2. Start by applying a thin layer of mascarpone cocoa covering frosting with an offset spatula to trap any crumbs.
3. Then apply a new layer of covering frosting (use acrylic discs for the outer smoothing) and smooth it with a steel frosting smoother.
4. Place the cake in the freezer for 30 minutes before removing the top acrylic disc.
5. Heat the chocolate, heavy cream, and black cocoa in the microwave in 20-second intervals until you get a flowing chocolate glaze.
6. Let it cool slightly before pouring it over the top of the layer cake.
7. Decorate the top of the cake with the remaining mascarpone frosting using a piping bag fitted with a 1b nozzle.
8. Serve immediately or refrigerate until ready to serve.