Preheat oven to 425°F. Line a 12-count muffin tin with liners or spray with nonstick spray.
In a medium bowl, whisk together flour, baking powder, baking soda, salt, cinnamon, and nutmeg.
In a large bowl, mash bananas until mostly smooth. Stir in melted butter and brown sugar until combined. Add egg, vanilla, and milk and mix until smooth.
Add dry ingredients to the banana mixture and stir just until combined. Do not overmix.
Fold in walnuts.
Divide batter evenly among muffin cups, filling to the top.
In a small bowl, combine topping ingredients until crumbly. Sprinkle evenly over muffins.
Bake 5 minutes at 425°F. Without opening the oven, reduce the temperature to 350°F and bake 14–16 minutes more, until a toothpick inserted in the center comes out clean.
Cool in pan 5 minutes, then transfer to a wire rack.
Notes
For deeper flavor, walnuts can be lightly toasted at 350°F for 5–7 minutes before using.Use very ripe bananas for the best banana flavor.Store in an airtight container up to 3 days or freeze up to 3 months.