Spray a 9x5 inch loaf pan with cooking spray and line the bottom with parchment paper. Preheat your oven to 350°.
In a large bowl, beat the butter and brown sugar with an electric hand mixer for about 2 minutes, or until fluffy.
Use a potato masher or fork to mash the bananas. Add the mashed bananas, eggs, sour cream, and vanilla to the sugar mixture. Beat until thoroughly combined.
Whisk together the flour, soda, and salt in a small bowl. Add to the wet ingredients and stir just until combined.
Pour half of the batter into the prepared loaf pan, then drizzle half of the Nutella on top of the batter. Repeat with the remaining batter and rest of the Nutella. Try not to let much of the Nutella touch the sides of the pan; it tends to burn.
Bake the bread in a preheated oven at 350° for 30 minutes. Turn the heat down to 325°; bake for another 30 minutes, or until no wet batter comes out on a toothpick inserted in the center.
Let the bread cool in the pan for 5-10 minutes, then carefully invert onto a cooling rack. Bread slices easier after it is completely cool, but it is simply delicious warm!