Melt the chopped semi-sweet chocolate over a double boiler, stirring until smooth. Let it cool slightly.

In a separate bowl, whip 1 cup of heavy cream until soft peaks form. Set aside.

In another bowl, whisk the egg yolks and sugar together until pale and fluffy. Stir in the melted chocolate and vanilla extract.

Gently fold the whipped cream into the chocolate mixture until combined.

Beat the egg whites in a clean bowl until stiff peaks form. Fold the egg whites into the mousse mixture carefully. Refrigerate the mousse while you prepare the cake.

Preheat your oven to 350°F (175°C). Grease and flour a 9-inch round cake pan.

In a bowl, whisk together the flour, cocoa powder, baking powder, and salt.

In a large bowl, beat the softened butter and sugar until light and fluffy. Gradually add eggs, mixing well after each addition, then stir in the vanilla extract.

Gradually add the dry mixture and buttermilk alternately until all combined.

Pour the batter into the prepared cake pan and bake for 30-35 minutes, or until a toothpick inserted in the center comes out clean. Let it cool in the pan for 10 minutes before transferring it to a wire rack.

Once the cake is completely cool, slice it in half horizontally. Place one layer on a cake platter and spread half of the chocolate mousse over it.

Top with the second cake layer and the remaining mousse.

Cover the cake with plastic wrap and refrigerate for at least 4 hours, or overnight if possible.

Before serving, whip the remaining cup of heavy cream to soft peaks and spread it over the top. Garnish with chocolate shavings or a dusting of cocoa powder.