Mash the bananas with a fork
Chop the butter
Mix banana and butter in a bowl
Mix flour and baking powder
Mix banana/butter, flour/baking powder, sugar, and vanilla in a large bowl
Chop the chocolate (not too finely) and add it as well
Put cupcake liners in a muffin pan
Pour dough into each cupcake liner till they are each full
Bake the muffins in the oven at 175° C for 25-30 min