Preheat Oven: Set oven to 350°F (175°C). Grease and flour a 9x13-inch baking pan.
Mix Dry Ingredients: Whisk together flour, baking soda, and salt in a medium bowl.
Cream Butter and Sugar: Beat butter and sugar until light and fluffy.
Add Eggs and Vanilla: Add eggs one at a time, then stir in vanilla extract.
Mix in Bananas and Sour Cream: Combine mashed bananas and sour cream into the mixture.
Combine Wet and Dry: Gradually add dry ingredients to wet, stirring until just combined.
Layer Batter and Nutella: Pour half the batter into the pan. Drop spoonfuls of Nutella and swirl gently with a knife. Repeat with remaining batter and Nutella.
Bake: Bake for 35–40 minutes or until a toothpick comes out clean.
Cool and Serve: Cool in pan for 10 minutes, transfer to wire rack. Dust with powdered sugar and chopped hazelnuts if desired.