Mash the bananas, mix with the yogurt and stir in the margarine and eggs. Mix the flour, baking powder and vanilla sugar and fold into the yogurt mixture with the cinnamon and sweetener.
Fill half of the dough into 12 muffin tins, place a teaspoon of Nutella on each and cover each with a tablespoon of dough.
Bake in a preheated oven at 200 ° C for about 20-25 minutes.