We mix the butter at room temperature together with the sugar and vanilla sugar until it becomes like a paste.
Beat the egg whites together with a pinch of salt, then gradually add the creamy butter and sugar composition.
We mix the flour with the baking powder, then we incorporate it in the composition, but this time we will mix it lightly with a spoon.
We put the composition in a form (I have silicone so I didn't even need to grease it).
We break the chocolate into pieces and put it on top of the composition. Bake for about 45 minutes, until it passes the toothpick test.