Sieve the flour, baking powder and custard powder into a bowl, then mix with sugar and vanilla sugar. Add eggs and butter or margarine and carefully stir all ingredients with a whisk or mixing spoon.
Pour half of the batter into the 12 hollows of a prepared muffin tin. Mix the remaining dough with cocoa powder and milk and spread on the light dough.
Bake the muffins at 180 ° C in the preheated oven for about 25 minutes.
Then let the muffins stand in the mold for about 10 minutes, then remove them from the mold and let them cool on a cake rack.
Dust the muffins with powdered sugar before serving.