Preheat oven to 375 degrees. Spray or line a muffin tin.
Add the bananas, brown sugar, and sugar to a mixing bowl and beat on medium speed until well combined.
Beat in the egg, butter, and vanilla until just combined.
Add the flour, baking powder, baking soda, cinnamon, and salt to a small bowl and stir to combine.
Add the flour mixture to the wet mixture, stirring until just combine. Do not overmix.
Stir in the walnuts.
Spoon the mixture into the muffin tin 2/3 of the way full.
Bake for 18 minutes or until a tester comes out clean.
Notes
Bananas should be very ripe for the best banana muffins. Bake yellow bananas at 300 degrees for 15-30 minutes, until very browned, to ripen the bananas quickly. Cool completely before using.Swap the walnuts for pecans or chocolate chips, if preferred.Store tightly covered at room temperature for 4-5 days.