Preheat the oven to 425°F. Line 12 muffin cups with liners, set aside.
In a large bowl, whisk together the dry ingredients: flour, baking powder, baking soda, cinnamon, and salt. Set aside.
1 ½ cups all-purpose flour, 1 teaspoon baking powder, 1 teaspoon baking soda, ½ teaspoon ground cinnamon, ¼ teaspoon salt
Whisk together the mashed bananas, sugar, egg, and vanilla in a medium bowl. Pour the wet ingredients into the dry and stir until combined.
1 ½ cups mashed bananas, ½ cup light brown sugar, 1 large egg lightly beaten, 1 teaspoon vanilla extract
Stir in the walnuts
1 cup chopped walnuts
Fill each muffin liner with the batter.
Let the batter sit in the liners for 15 minutes.
Bake muffins for 5 minutes at 425°F. Lower the oven temperature to 350°F and bake for 10-15 minutes more, until a toothpick inserted in the center comes out clean.
Let cool for 5 minutes in the muffin tin, take the muffins out, and cool completely on a wire rack.
Notes
Use overripe bananas. Brown-spotted = maximum sweetness.
Mash bananas smooth. Large chunks create wet pockets in the muffin batter.
Don’t overmix the flour mixture. Stir until no dry streaks. Then stop for moist muffins.
Rest filled muffin cups. Those 15 minutes give you higher domes and fewer sad muffins.
Rotate the pan halfway. Even home ovens have hot spots; a quick spin means uniform color.
Cleanup hack. Line the cooling wire rack with parchment for crumb-catching magic (less time with the broom).