Preheat the oven at 220ºC/425ºF conventional oven and line a cupcake trays with 6 cupcake liners, to only line every other cupcake hole, as it helps them to dome up.
In a bowl, sift the all-purpose, baking powder, baking soda and salt together. Give it a mix to make sure it is all evenly mixed together. Mash the banana and set it aside.
175 g all-purpose flour, 1 teaspoon baking powder, ¼ teaspoon baking soda, ¼ teaspoon salt
In another mixing bowl add the melted butter, granulated sugar and the egg. Whisk it together until combined.
60 g butter, 100 g granulated sugar, 1 large eggs
Then add the sour cream, banana and vanilla extract and whisk it together until combined.
50 g sour cream or full-fat plain greek yogurt, 200 g very ripe banana, 1 teaspoon vanilla extract
Add the flour mixture to the wet batter and gently fold it together with a spatula.
First divide half of the cake batter into the 6 cupcake liners. Then add a teaspoon of Nutella in the middle of the batter. Add the other half of the batter on top. Finish them off with another teaspoon of Nutella on top the batter and swirl it in with a spoon or knife.
60 g Nutella
Bake them for 5 mins and then reduce the heat to 175ºC/350ºF and further bake for 11-13 min. Let the muffins cool in the hot pan. After 3-5 min, gently remove the muffins from the pan and let them cool down completely.
Serve it on the day, but they can also last well for a few days on the kitchen counter, in an airtight container.