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