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, beaten egg, melted unsalted butter, and vanilla extract. Mix until smooth.
Gradually add the dry ingredients to the banana mixture, stirring 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 muffin cups.
Use a toothpick or skewer to 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 pan for 5 minutes.
Transfer the muffins to a wire rack to cool completely before serving.