Roughly chop the chocolate. Peel the skin of a banana, crush it with a fork and mix in lemon juice. Cream fat, sugar, salt and vanilla sugar with the whisk of the hand mixer. Stir in the eggs one by one. Mix flour and baking powder and stir in alternately with 3 tbsp. milk. Halve the dough, stir in one half cocoa, 2 tablespoons milk and chocolate.
Stir banana into the second half of the dough. Spread the chocolate dough evenly into the hollows (12 pieces) of a greased muffin tray sprinkled with flour. Spread banana dough evenly on top and marble with a small fork. Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for approx. 25 minutes. Remove from the oven, place on a cake rack and after about 30 minutes carefully remove from the trays, place on a cake rack and allow to cool. Just before serving, cut 1 banana into slices. Cover muffins with banana slices and decorate with chocolate sauce
Waiting time approx. 1 hour