Preheat Oven: Preheat your oven to 350°F (175°C). Line a muffin tin with paper liners or grease with cooking spray.
Mix Dry Ingredients: In a medium bowl, whisk together the flour, baking powder, baking soda, and salt.
Combine Wet Ingredients:
In a large mixing bowl, combine the mashed bananas, granulated sugar, melted butter, eggs, and vanilla extract. Mix until smooth.
Combine Wet and Dry:
Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
Add Nutella:
Warm the Nutella slightly in the microwave to make it easier to swirl. Add dollops of Nutella to the batter and gently swirl with a knife or toothpick to create a marbled effect.
Fill Muffin Tin:
Divide the batter evenly among the muffin cups, filling each about 2/3 full.
Bake:
Bake in the preheated oven for 18-20 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Cool:
Allow the muffins to cool in the tin for 5 minutes before transferring to a wire rack to cool completely.
Serve:
Enjoy your delicious Nutella Banana Swirl Muffins warm or at room temperature!