Preheat your oven to 350°F (175°C). Mix dark chocolate, butter, sugar, egg yolks, and vanilla until smooth. Pour into a greased pan and bake for 25-30 minutes. Cool completely.

For the dark chocolate mousse, melt dark chocolate. Whip heavy cream to soft peaks, fold into chocolate, then fold in beaten egg whites. Chill for at least 30 minutes.

For the milk chocolate mousse, melt the milk chocolate. Whip heavy cream to soft peaks, fold into chocolate, then fold in beaten egg whites. Chill for another 30 minutes.

For the white chocolate mousse, melt the white chocolate. Whip heavy cream, fold into chocolate, then fold in egg whites. Chill for another 30 minutes.

Assemble the cake: place cooled cake on a platter and layer dark mousse, then milk mousse, then white mousse, chilling in between layers.

For the ganache, heat cream and pour over chocolate mixture until smooth. Let cool slightly, then pour over the mousse layers. Chill for at least 2 hours.

Slice with a warm knife and serve with optional whipped cream or berries.