Preheat the oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners.
In a medium bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt.
In a large bowl, combine the mashed bananas, granulated sugar, egg, melted butter, and vanilla extract. Mix until smooth.
Add the dry ingredients to the wet ingredients and stir until just combined. Do not overmix.
Spoon half of the batter evenly into the prepared muffin cups.
Add about 1 teaspoon of Nutella on top of the batter in each muffin cup.
Spoon the remaining batter over the Nutella layer, dividing evenly among the cups.
Add another teaspoon of Nutella on top of each muffin and use a toothpick or skewer to gently swirl the Nutella into the batter.
Bake for 18-20 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Remove the muffins from the oven and allow them to cool in the tin for 5 minutes before transferring to a wire rack to cool completely.