Mix the flour, baking powder, salt and custard powder together.
Melt the butter in a water bath until it has liquefied. Now add sugar, vanilla sugar, milk and eggs to the butter one after the other and mix well with a whisk.
Then add the butter mixture and the chopped chocolate to the flour and stir the mixture into a smooth dough.
Pour the batter into the molds. Bake at 190 ° C (fan oven) for about 20 minutes.