Preheat oven to 350 degrees. Line a 12-count muffin pan with cupcake liners.
In a bowl, stir together the flour, baking soda, baking powder, and salt. Set aside. In another bowl, stir together the mashed bananas, sugar, brown sugar, egg, oil, buttermilk, and vanilla extract. Add the wet ingredients to the dry ingredients and stir until completely combined. Divide the batter evenly into the muffin cups.
Place the ½ cup of Nutella into a microwave safe bowl and heat 15-30 seconds, or until Nutella is smooth and stir-able. Spoon about two teaspoons of Nutella on top of each muffin. Using a toothpick, swirl the Nutella into the top of the batter, using a circular motion. Repeat with all of the muffins.
Bake 15-18 minutes, or until baked through and set.