How to Make It
Preheat oven to 350°F. Grease and line a standard loaf pan with parchment paper; set aside.
In a large bowl, mash bananas until relatively smooth and liquid. Add sugar, oil, and vanilla; stir to combine. In a separate bowl, whisk together flour, baking powder, baking soda, and salt. Stir dry ingredients into banana mixture until no pockets of flour remain.
Pour half of the batter in prepared loaf pan. Evenly dollop 3 tablespoons Nutella across the surface of batter in several clumps. Run a butter knife through the batter and Nutella, creating a swirling pattern. Pour remaining batter on top. Add remaining 3 tablespoons Nutella on top of the batter and repeat swirling process.
Bake until a toothpick inserted into the center of the loaf comes out clean, about 1 hour. Allow to cool before removing from pan and slicing.