Preheat oven to 350°. Line a 9x5 inch loaf pan with parchment paper and spray lightly with cooking spray.

Cream together butter and sugar the bowl of a mixer or large mixing bowl if doing by hand.

Add eggs, one at a time. Mix until combined, it will look curdled. Add vanilla. Mix until well combined.

Add mashed bananas and milk.

Sieve flour, baking soda, and salt over the bowl.

Gently mix until flour can no longer be seen in the batter.

Microwave Nutella for 15 to 20 seconds, or until it loosens. Add 1 cup of batter and mix to combine.

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 to one hour. A toothpick will come out with some crumb. Let cool in pan for 15 minutes, then remove and let cool completely on a wire rack.