Preheat the oven to 400F degrees and line a muffin pan with papers.
In a large bowl whisk together the flour, baking soda, baking powder, cinnamon and salt.
In a separate bowl, whisk the sugars, melted butter, eggs, vanilla extract and sour cream.
Stir in the mashed bananas.
Gently fold the flour mixture into the dry ingredients followed by the chopped walnuts.
Spoon the batter into the prepared muffin tin.
Bake for 5 minutes at 400F degrees. Without removing the pan from the oven, turn the oven down to 375F and continue baking for 10-12 minutes or until an inserted toothpick comes out clean.
For less sweet muffins, use ½ cup white sugar and ¼ cup brown sugar.3 large bananas should be about 1 ½ cups mashed. Muffins are best enjoyed the same day that they're baked. Store leftovers in an airtight container at room temperature for up to 3 days.