Preheat oven to 350 degrees. Line a loaf pan with parchment paper.
Stir the sugars and melted butter together until well combined. Add the eggs, vanilla, and mashed bananas, then mix until everything is fully blended and smooth.
Stir in flour, salt and baking soda just until no flour remains. Batter will be thick.
Pour half of the batter into the prepared loaf pan. Smooth the top.
Microwave the Nutella for about 20–25 seconds until slightly softened. Spoon half of it evenly over the batter in small dollops, then use a knife to gently swirl it through the batter to create a marbled effect.
Pour the remaining batter into the pan, spreading it evenly. Add small dollops of the remaining Nutella on top, then use a knife to gently swirl it through the batter just as you did before.
Bake for 50–55 minutes, or until a cake tester or toothpick inserted into the center comes out with just a few moist crumbs attached.
Let cool in the pan for 15 minutes before transferring to a wire rack to cool the rest of the way.
