Puree the banana with the sugar, vanilla sugar and 50 ml milk until the mixture is creamy. Then add the rest of the milk and the oil and mix well with the mixer. Mix flour with baking powder and cocoa powder and then add to the banana mixture. Melt the chocolate and mix in. Roughly chop the walnuts and fold them into the mixture.
Divide into 12 muffin tins and bake at 200 ° C for about 20 minutes.