Preheat oven to 350 degrees and grease a 9x5 loaf pan.
Cream together the butter, sugar, eggs, and vanilla.
Stir together the flour, baking soda, cinnamon, and salt.
Add dry ingredients to butter mixture and mix well. Fold in the bananas, sour cream, and nuts until just combined.
Pour half of the batter into the prepared loaf pan. Drop spoonfuls of the Nutella over the top of the batter. Cover with the remaining batter. Use a knife to swirl the batter around in the loaf pan.