Step 1: Preheat Oven & Prepare Pans
Preheat oven to 350°F (175°C). Grease and line three 8-inch round cake pans with parchment paper.

Step 2: Make the Chocolate Sponge Cake

1. In a bowl, sift together flour, cocoa powder, baking powder, and salt.

2. In a large bowl, beat eggs and sugar for about 5 minutes until pale, thick, and fluffy.

3. Mix in milk, oil, and vanilla extract.

4. Gently fold in the dry ingredients until just combined.

5. Divide batter evenly between the prepared pans.

6. Bake for 20–25 minutes, or until a toothpick inserted in the center comes out clean.

7. Let cakes cool in pans for 10 minutes, then transfer to a wire rack to cool completely.

Step 3: Make the Swiss Chocolate Buttercream

1. In a heatproof bowl over a pot of simmering water, whisk together egg whites and sugar.

2. Heat until the sugar is dissolved and the mixture reaches 150°F (65°C), stirring constantly.

3. Remove from heat and whip on high speed until stiff peaks form and mixture is cooled to room temperature.

4. Gradually add butter, one piece at a time, beating until smooth and fluffy.

5. Add melted chocolate and vanilla, then beat until fully combined.

Step 4: Assemble the Cake

1. Place one sponge layer on a cake stand or plate. Spread a layer of buttercream on top.

2. Repeat with remaining layers. Cover the top and sides of the cake with a smooth layer of buttercream.

3. Decorate with chocolate curls, shavings, or a drizzle of ganache if desired.

Step 5: Chill & Serve
Chill the cake for at least 30 minutes before slicing for clean, firm layers.