Preheat your oven to 350°F (175°C) and grease a muffin tin or line it with paper liners.
In a large mixing bowl, combine the mashed bananas and melted butter, mixing them until well blended.
Stir in the sugar, beaten egg, and vanilla extract until fully incorporated.
Sprinkle the baking soda and salt over the mixture, then mix well.
Gradually add the all-purpose flour, folding it into the mixture gently until no dry flour remains. Do not overmix!
If using, fold in the chopped walnuts or pecans for added texture and flavor.
Divide the batter evenly among the muffin cups, filling each about 2/3 full.
Using a teaspoon, drop dollops of Nutella on top of each muffin batter. Use a toothpick or a skewer to swirl the Nutella into the batter, creating a marbled effect.
Bake for 18-20 minutes, or until a toothpick inserted into the center comes out clean.
Allow the muffins to cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.
Notes
Serve warm with powdered sugar and extra Nutella for dipping.