Preheat oven to 350°F. Line two 12-count muffin tins with 18 paper liners. Set aside.
In a medium bowl, whisk the flour, baking soda, baking powder, and salt together. Set aside.
2 cups all-purpose flour, 1 teaspoon baking soda, ½ teaspoon baking powder, ½ teaspoon kosher salt
Using a hand mixer, beat the butter, bananas, brown sugar, sugar, eggs, and vanilla together on medium speed for 3 minutes. Gently stir in the flour mixture until just combined. Set aside.
½ cup unsalted butter, 6 ripe bananas, ½ cup brown sugar, ¼ cup granulated sugar, 2 large eggs, 1 teaspoon pure vanilla extract
Place the Nutella in a small microwave-safe bowl and warm for 30 seconds until it's thinned.
⅓ cup Nutella
Fill the wells of the muffin tins so they are ¾ full. Spoon the Nutella over the batter, and swirl them together with a knife.
Bake for 15-18 minutes, until a toothpick is inserted in the center of the loaf and it comes out clean.
Move the muffins over to a cooling rack and allow them to cool for 15 minutes.