Preheat oven to 350 degrees. Line a loaf pan with parchment paper.
Stir together sugars and melted butter until combined. Mix in 2 eggs, vanilla and mashed bananas, stirring well.
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. Dollop ½ of the Nutella evenly over the batter. Use a knife to swirl the Nutella into the batter.
Pour the rest of the batter into the pan. Dollop the remaining Nutella over the top of the batter and swirl into the batter the same way as above.
Bake for about 50 minutes until a cake tester or toothpick comes out with moist crumbs remaining.
Let cool in the pan for 15 minutes before transferring to a wire rack to cool the rest of the way.