Take the whites out of the egg yolks, and whisk them until they are stiff.
Add sugar to the yolk and mix well. Add the flour, the baking powder and a pinch of salt, and mix well.
Add the melted butter, then delicately add the egg whites to the batter.
Divide the batter in half. Add in one half the vanilla sugar, and the melted chocolate in the other half.
In a cake mould, pour the batter, alternating between the vanilla and the chocolate batter. Start with vanilla, as chocolate has a tendancy to burn.
Bake 50 minutes at 320°F (160°C).
There you are, your marble cake is ready !