Preheat oven to 350°. Spray three small loaf pans (disposable are even better!) with cooking spray.
Mix oil, peanut butter, and sugar in a large bowl with a stand mixer (or a hand mixer is fine too).
Add the bananas, eggs, sour milk, and baking soda to the peanut butter mixture. Beat until combined.
Slowly mix in flour, beating just until combined. Try not to overmix!
Place about 3/4 cup of batter in the bottom of each pan and spread to fill. Place the Nutella and Fluff in separate bowls and warm for a few seconds in the microwave, until it’s spreadable. Drop spoonfuls of the Nutella and Fluff on top of the batter in each loaf. Then spread the rest of the batter evenly on top (about 1/4 cup per loaf).
Place pans on a cookie sheet (in case the fluff spills over!) and bake for about 20 minutes, until a toothpick comes out with only the slightest crumb attached. (I cooked it for about 24 minutes and the bottom was too done.)
Notes
This can be made as one loaf, but it would need to cook longer.