Preheat oven to 350F (175C) and lightly grease the sides and bottom of a 9x5 (23x13cm) bread pan. Set aside.
Combine butter and sugar in a large mixing bowl and use an electric mixer to beat together until well creamed.
Add well-mashed bananas and stir until well-combined.
Add eggs, one at a time, beating well after each addition.
Stir in sour cream and vanilla extract.
In a separate, medium-sized mixing bowl, whisk together flour, baking soda, and salt.
Use a spatula to gently fold dry ingredients into wet until just combined, taking care to not overmix.
Pour about half of the batter into the prepared 9x5” (23x13cm) bread pan and spread into an even layer.
Drizzle ⅓ cup of Nutella over the layer and use a butter knife to gently swirl into the batter.
Pour the remaining batter evenly into the bread pan. Drizzle the remaining 2 Tablespoons of Nutella over the surface and use a clean knife to swirl.
If desired, sprinkle the top of the Nutella banana bread with sugar.
Transfer to center rack of 350F (175C) preheated oven for 65-70 minutes, until a wooden skewer inserted into the center comes out clean or preferably with a few moist crumbs.
Allow bread to cool in its pan for 15 minutes before carefully removing to a cooling rack to cool completely before enjoying.
