Preheat oven to 400°. Line 12 (2½-inch) muffin cups with paper baking cups or generously spray with nonstick spray.

Whisk together flour, granulated sugar, brown sugar, baking powder, cinnamon, and salt in a medium bowl until combined. Make a well in center; set aside.

Whisk together egg, milk, and oil in small bowl. Stir in mashed banana. Add banana mixture all at once to flour mixture; stir just until moistened; do not overmix (batter will be lumpy). Fold in walnuts.

Spoon batter evenly into prepared muffin cups, filling each about ¾ full. Bake for 16 to 22 minutes or until toothpick inserted near centers comes out clean. Cool in muffin cups on a wire rack for 5 minutes. Remove from muffin cups. Cool slightly on wire rack; serve warm.