1. Preheat oven to 350ºF. Line a 12-cup muffin tin with paper liners or coat with cooking spray.
2. In a bowl, combine flours, baking powder and salt.
3. In another bowl, using an electric mixer on medium-high speed, beat butter and sugar until light, about 3 minutes. Beat in eggs, bananas and vanilla until smooth. Gradually stir in flour mixture. Fold in nuts.
4. Divide batter among muffin cups. Spoon 1 teaspoon Nutella on top of each cup. Swirl together Nutella and batter with a toothpick.
5. Bake until muffins are golden, about 22 minutes. Let cool in pan for 5 minutes, invert onto wire rack, turn muffins right side up and let cool completely.