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. Set aside.
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 banana muffin 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 to cover it.
Using a toothpick or skewer, gently swirl the Nutella into the batter to create a marbled effect.
Bake for 18 to 20 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Remove the muffins from the oven and let them cool in the tin for 5 minutes.
Transfer the muffins to a wire rack to cool completely before serving.