Preheat oven to 350 degrees. Line a 12-count muffin pan with liners or spray with non-stick spray.
In a bowl, combine the flour, baking soda, baking powder, and salt and set aside.
In another bowl, combine the mashed bananas, sugar, brown sugar, egg, oil, buttermilk, and vanilla extract and stir until well combined.
Add the wet ingredients to the dry ingredients and and mix gently until just combined.
Using an ice cream scoop, drop the batter evenly into the muffin cups, or if you don't have a scooper, fill muffin tins almost to the top of the muffin cup tins.
Place the ½ cup of Nutella into a microwave safe bowl and heat 15-30 seconds, or until Nutella is smooth and easy to stir.
Divide batter among the muffin cups. Spoon approximately 2 teaspoons of Nutella on top of each cup and swirl together Nutella and batter with a toothpick.You can also pipe or add some extra Nutella to the center of the muffins, if you want more inside. Bake until muffins are golden, about 17-20 minutes, check them after 17 minutes.
Let cool in pan for 5 minutes, invert onto wire rack, turn muffins right side up and let cool completely. (Or eat warm!)