01 - Preheat oven to 350°F. Line a 12-cup muffin tin with paper liners.
02 - In a large bowl, whisk together flour, baking powder, baking soda, and salt until well blended.
03 - In a separate bowl, whisk melted butter and sugar until smooth. Add eggs one at a time, beating thoroughly after each addition. Stir in mashed bananas and vanilla extract.
04 - Gently fold wet ingredients into dry ingredients until just combined. Avoid overmixing to prevent dense muffins.
05 - Divide batter evenly among prepared muffin cups, filling each approximately two-thirds full.
06 - Drop about 1 teaspoon of warmed Nutella onto each muffin. Use a toothpick or skewer to create swirl pattern by gently circling through the batter.
07 - Bake for 18 to 22 minutes, or until a toothpick inserted into the center comes out clean except for Nutella traces.
08 - Let muffins cool in pan for 5 minutes, then transfer to wire rack to cool completely before serving.