Melt dark chocolate over a bain-marie until smooth. Add half of butter and incorporate until well blended. Repeat this step with the white chocolate in another bowl.

Whip egg whites until foamy with a mixer and gradually add the sugar whipping the whole time until it holds very stiff peaks. Split the whites in 2 bowls.

Beat the yolks with your chosen flavor ingredients, split in half and add to each of chocolate mixtures. Remove from heat. 

Fold in a spoonful of the whites into each of chocolate mixtures then pour each chocolate into the remaining whites and fold gently until well combined. 

The white mousse will not be as thick as the dark mousse so first pour the dark mousse into serving bowls, then pour the white mousse and finish with dark mousse on top. Wrap with plastic and refrigerate few hours or overnight. Sprinkle with grated chocolate before serving.