Step 1: Prepare the Chocolate Sponge Cake

1. Preheat the oven: Begin by preheating your oven to 180°C (350°F). Grease and flour two 8-inch round cake pans to prevent sticking.
2. Whisk the eggs and sugar: In a large mixing bowl, whisk together the 4 eggs and 120 g of sugar until the mixture becomes light and fluffy. This will take about 5-7 minutes using an electric mixer.
3. Add the wet ingredients: Once the egg mixture is ready, gently fold in 80 g of sour cream and 100 g of vegetable oil until well combined.
4. Combine dry ingredients: In a separate bowl, sift together 175 g of flour, 55 g of cocoa powder, 12 g of baking powder, and 2 g of salt.
5. Mix dry and wet ingredients: Gradually add the dry ingredients to the wet mixture, folding gently until no dry flour is visible. Be careful not to overmix, as this can make the cake dense.
6. Bake the cake: Divide the batter evenly between the prepared cake pans. Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted into the center comes out clean.
7. Cool the cakes: Once baked, remove the cakes from the oven and allow them to cool in the pans for 10 minutes. Then, transfer them to a wire rack to cool completely.

Step 2: Prepare the Cream Filling

1. Mix the cream filling: In a mixing bowl, combine 400 g of Mascarpone cheese, 100 g of sour cream, 250 g of condensed milk, and 55 g of cocoa powder. Whisk until the mixture is smooth and creamy.
2. Chill the filling: Place the cream filling in the refrigerator while you prepare the frosting, allowing it to thicken slightly.

Step 3: Make the Frosting

1. Melt the chocolate: In a heatproof bowl, melt 100 g of dark chocolate together with 40 g of milk and 60 g of butter. You can do this using a microwave in short bursts or over a double boiler until smooth.
2. Cool the frosting: Allow the melted chocolate mixture to cool slightly before using it to frost the cake.

Step 4: Prepare the Interlayers

1. Make the syrup: In a small saucepan, combine 100 g of water and 50 g of sugar. Heat over medium heat until the sugar dissolves. Remove from heat and stir in 18 g of Cognac or Brandy if using.

Step 5: Assemble the Cake

1. Level the cakes: Once the chocolate sponge cakes have completely cooled, level the tops using a serrated knife to create even layers.
2. Soak the cake layers: Place one cake layer on a serving platter and brush the top with the syrup you prepared. This adds moisture and flavor.
3. Add cream filling: Spread half of the cream filling over the first layer. Place the second layer on top and repeat the process, brushing with syrup and spreading the remaining cream filling.
4. Top with the final cake layer: Place the final layer of cake on top. If desired, you can cover the top with the remaining cream filling or leave it as is.

Step 6: Frost the Cake

1. Apply the chocolate frosting: Use a spatula to spread the cooled chocolate frosting evenly over the top and sides of the cake.
2. Decorate: If desired, garnish the top with chocolate shavings, cocoa powder, or fresh berries for an extra touch.

Step 7: Chill and Serve

1. Chill the cake: Allow the cake to chill in the refrigerator for at least 1 hour before slicing. This helps the flavors meld and makes for cleaner slices.
2. Slice and enjoy: When ready to serve, slice the cake into wedges and enjoy!