01 - Preheat your oven to 350°F. Line a 12-cup muffin tin with paper liners.
02 - In a large bowl, whisk together the flour, baking powder, baking soda, and salt until well combined.
03 - In a separate bowl, whisk the melted butter and sugar until smooth. Add eggs one at a time, beating well after each addition. Stir in the mashed bananas and vanilla extract.
04 - Fold the wet ingredients into the dry ingredients until just combined. Do not overmix—some small lumps are fine.
05 - Divide the batter evenly among the muffin cups, filling each about 2/3 full.
06 - Drop about 1 teaspoon of Nutella onto each muffin. Use a toothpick or skewer to gently swirl the Nutella into the batter, creating a marbled effect.
07 - Bake for 18 to 22 minutes, or until a toothpick inserted into the center comes out clean (except for some Nutella residue).
08 - Let the muffins cool in the pan for 5 minutes, then transfer to a wire rack to cool completely before serving.