Preheat oven to 400°F (200°C). Line a 12-cup muffin tin with paper liners or spray with non-stick spray.

In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.

In a large bowl, mash the bananas. Whisk in the melted butter, granulated sugar, and brown sugar until combined.

Whisk the egg and vanilla extract into the wet banana mixture.

Gently fold the dry ingredients into the wet ingredients using a spatula until just combined. Do not overmix; the batter should be slightly lumpy.

Warm the Nutella in the microwave for 15-20 seconds until easily spreadable but not hot.

Spoon the muffin batter into the prepared tin, filling each cup about 3/4 full.

Drop a generous teaspoon of warmed Nutella onto the top of each muffin cup. Use a toothpick to gently swirl the Nutella into the batter in a figure-eight motion.

Bake at 400°F (200°C) for 5 minutes. Without opening the oven door, reduce temperature to 350°F (175°C) and bake for another 12-15 minutes, until a toothpick comes out mostly clean.

Let the muffins cool in the pan for 5 minutes before transferring to a wire rack to cool completely.