Preheat the oven to 350° F. Grease or spray a regular sized muffin pan or line a regular sized muffin pan with liners.

In a medium bowl, combine flour, baking soda, baking powder, and salt. Set aside.

In a separate medium bowl, mash peeled bananas. Mix in sugars. Stir in melted butter, egg, sour cream, and vanilla extract.

Slowly whisk the dry flour mixture into the wet banana mixture until combined. Do not overmix.

Pour the muffin batter into the muffin pan until the cups are about 3/4 full.

Scoop Nutella spread into a small bowl and heat for 10-20 seconds in the microwave. Using a small spoon, drizzle a small amount of Nutella (about 1 teaspoon) over the banana muffin batter. Use a toothpick or skewer to gently swirl and combine the Nutella with the muffin batter on top of the muffin.

Bake muffins for 15-20 minutes (depending on how full your cups are) until a toothpick or skewer inserted in center comes out clean. Allow muffins to cool in muffin pan for 10 minutes and then set aside to cool on wire rack.