Preheat your oven to 350°F (175°C).
Grease a 9x5 inch loaf pan with cooking spray or line it with parchment paper.
In a large mixing bowl, combine the sugar, vegetable oil, and peanut butter. Mix until well combined. Add in the ripe bananas and mash them into the mixture. Then, mix in the eggs and sour milk until everything is smooth and creamy.
In a separate bowl, whisk together the baking soda and sifted all-purpose flour. Gradually add the dry mixture to the wet ingredients, folding gently until just combined.
Warm the Nutella and marshmallow fluff slightly in the microwave for about 15 seconds. Spread half of the batter into the prepared loaf pan. Drizzle half of the warmed Nutella and marshmallow fluff over it. Repeat with the remaining batter and toppings.
Place the loaf pan in the preheated oven and bake for 55-65 minutes, or until a toothpick inserted in the center comes out clean.
Once baked, let it cool in the pan for about 10 minutes before transferring to a wire rack to cool completely.
