For the Cake:

Preheat the oven to 350°F (175°C). Grease and line two 8-inch cake pans with parchment paper.

Mix dry ingredients: In a medium bowl, whisk together flour, cocoa powder, baking powder, and salt.

Cream butter and sugar: In a large bowl, cream the butter and sugar until light and fluffy. Add eggs one at a time, beating well after each addition. Stir in the vanilla extract.

Combine wet and dry: Add half of the flour mixture to the butter mixture, followed by the milk, then the remaining flour mixture. Mix until just combined.

Bake: Divide the batter evenly between the two prepared pans and bake for 25 minutes or until a toothpick comes out clean. Let the cakes cool completely before assembling.

For the Chocolate Mousse:

Melt the chocolate: In a double boiler, melt the dark chocolate with butter. Stir until smooth and let it cool slightly.

Beat egg yolks: In a separate bowl, whisk the egg yolks with sugar until pale and creamy. Gradually add the melted chocolate, stirring constantly.

Whip the cream: In another bowl, whip the heavy cream until soft peaks form. Fold the whipped cream into the chocolate mixture until fully incorporated.

Beat egg whites: In a clean bowl, beat the egg whites until stiff peaks form. Gently fold the egg whites into the chocolate mixture, ensuring you don’t deflate the mousse.

Assembling the Cake:

Place one cake layer on a serving plate and spread a thick layer of chocolate mousse over the top.

Add the second cake layer and cover the entire cake with the remaining mousse.

Use a spatula to smooth the mousse on the sides and top of the cake.

For Decoration:

Garnish with chocolate shavings, whipped cream, and fresh fruits if desired.

Chill the cake in the refrigerator for at least 2 hours before serving to allow the mousse to set.