Mix the butter with the sugar and the orange-baking until creamy, then beat the eggs over it. Add the vanilla sugar, flour, baking powder and milk. Chop 150 g white couverture and add to the batter.
Pour the dough into a greased bundt cake pan and bake in a preheated oven at 160 degrees for 1 hour.
Melt the rest of the couverture in a water bath and cover the cake with it.