Preheat the oven to 175 ° C. Grease the form and sprinkle with breadcrumbs.
Beat butter, sugar, vanilla sugar and eggs with the whisk of the hand mixer until a creamy mixture is formed. The sugar should dissolve completely.
Mix the flour, cornstarch and baking powder. Stir alternately with the milk into the batter. Stir quickly, otherwise the flour will begin to swell and the dough will dry out. The dough should fall from the spoon with difficulty. If he does not do this, balance with flour or milk.
Divide the batter evenly between two bowls. Add the grated chocolate and cocoa to one half and stir in. First distribute the light-colored dough evenly on the base, then pour the dark dough over it and smooth it out. Marble the dough from bottom to top with a fork and smooth it out again.
Bake immediately on the middle rack for about 60 minutes. Chopstick sample!
Let it cool down a bit, but turn it out of the mold while it is still warm and dust with powdered sugar if you like.