0 Preheat oven to 350°F. Lightly grease a 9x5-inch metal loaf pan with cooking spray, then line it with parchment, leaving an overhang on the two long sides to create a sling.
1 In a large mixing bowl, mash the bananas with a fork until mostly smooth, and only a few lumps remain.
2 Whisk in the slightly cooled melted butter, brown sugar, and granulated sugar until combined. Mix in the eggs, vanilla, and sour cream.
3 In a separate bowl, whisk together the flour, baking soda, baking powder, and salt. Whisk the dry ingredients into the banana mixture until everything is fully incorporated, being careful not to overmix.
4 Pour about one-third of the batter into the prepared pan and spread it evenly. Microwave the Nutella for 20-25 seconds, just until warm and pourable. Transfer it to a piping bag or zip-top bag (snip a small corner) and drizzle one-third of it over the batter. Use a butter knife or toothpick to gently swirl.
5 Repeat the layering twice more: add another third of batter, drizzle another third of Nutella, and swirl. Finish with the final layer of batter and one last swirl of Nutella on top.
6 Bake at 350°F for 55-60 minutes or until a toothpick inserted in the middle comes out with just a few moist crumbs.
7 Allow the bread to cool in the pan for 15-20 minutes. Use the parchment sling to lift it out, then transfer to a wire rack to cool completely.
8 Slice and enjoy. Store the Nutella banana bread in an airtight container for up to 5 days.
