Start by preheating your oven to 350°F (175°C) and greasing your loaf pan with butter or a non-stick spray. Mash your ripe bananas in a bowl until smooth but still slightly chunky for texture.
In a large mixing bowl, use a hand mixer or stand mixer to cream together the softened butter, granulated sugar, and brown sugar until light and fluffy. This will help create a tender crumb in your bread.
Beat in the eggs one at a time, ensuring each is fully incorporated. Then mix in the mashed bananas and vanilla extract until combined. The mixture might look a bit thick, but that’s perfect.
In a separate bowl, whisk together the all-purpose flour, baking soda, and salt. Gradually add these dry ingredients to your wet banana mixture, folding gently with a spatula until just combined. Avoid overmixing to keep the bread tender.
Stir in the chopped walnuts, distributing them evenly throughout the batter for that added crunch.
Pour half of the batter into your prepared loaf pan. Dollop half of the Nutella over the batter, then use a knife or skewer to swirl it gently. Repeat with the remaining batter and Nutella, creating beautiful chocolate-hazelnut streaks throughout the loaf.
Bake in your preheated oven for 55-65 minutes, or until a toothpick inserted into the center comes out clean (a few Nutella streaks are okay, but no raw batter). Let the bread cool in the pan for 10 minutes before transferring it to a cooling rack to cool completely.