Preheat oven to 350°. Line a 9x5 inch loaf pan with parchment paper and spray lightly with cooking spray.
Cream butter and sugar. In the bowl of an electric mixer (or in a large bowl by hand), cream together butter and sugar.
Mix in wet ingredients. Add eggs, one at a time. Mix until combined, it will look curdled. Add vanilla. Mix until well combined.
Add mashed bananas and milk.
Add dry ingredients. Sieve flour, baking soda, and salt over the bowl.
Gently mix until flour can no longer be seen in the batter.
Microwave Nutella on HIGH for 15 - 20 seconds, or until it loosens. Add 1 cup of batter and mix to combine.
Create a marble swirl. Spoon both batters alternately into the prepared pan. Use a chopstick or knife and gently swirl the chocolate into the banana batter.
Bake for 50 minutes - 1 hour. A toothpick will come out with some crumbs. Let cool in the pan for 15 minutes, then remove and let cool completely on a wire rack. Slice and enjoy!