Preheat oven to 350 degrees Fahrenheit. Line a muffin pan with paper liners and set aside.
In a bowl, mash peeled bananas with a fork. Add granulated sugar, brown sugar, oil, eggs, milk, and vanilla extract. Stir until well combined. Set aside.
In a separate bowl, whisk (or sift) together the dry ingredients: flour, baking soda, baking powder, cinnamon, and salt. Make a well in the middle and pour wet ingredients into dry ingredients. Stir until just combined. Fold in walnuts.
Distribute batter between 15 muffin cups. Bake for 19-21 minutes, or until a toothpick inserted into the center of muffin comes out clean (or with crumbs, not batter).
Remove from oven and allow pan to cool for 2-3 minutes before removing muffins to a cooling rack to cool completely. Store cooled muffins in a sealed container.
Muffins will keep at room temperature in a sealed container for 3 days.
Baked muffins can be stored in the freezer for up to 3 months in a freezer safe container.
If using paper liners, allow your muffins to cool completely before unwrapping, or they will stick to the papers.