Preheat oven to 350°F. Line a 12-cup muffin tin with paper liners, spray the liners with cooking spray.
In a large bowl, mash the bananas. Whisk in the butter, milk, brown sugar, and vanilla extract. Set aside.
In a small bowl, whisk together the flour, baking powder, baking soda, cinnamon, nutmeg, and salt. Pour the dry ingredients into the wet ingredients and mix with a wooden spoon or spatula until just combined, don't overmix. Fold in the chopped walnuts.
Scoop the batter into the prepared muffin tin. Bake for 20-25 minutes, until toothpick inserted into center of a muffin comes out clean. Let the muffins sit in the pan for about 5 minutes before transferring them to a wire rack to finish cooling.