For the banana Nutella muffins, preheat the oven to 200 °C and place paper cups in the muffin tray. Peel the bananas and mash them.
In a bowl mix flour and baking powder, in a second bowl beat egg, sugar, banana puree, Nutella and buttermilk until foamy.
Slowly and briefly stir in flour mixture Pour batter into ramekins and bake for 20 to 25 minutes.