Preheat oven to 350°.
Cream butter and sugar in an upright mixer or large bowl.
Mix mashed bananas with 2 eggs and vanilla, then add to mixing bowl.
Combine flour, baking soda, cinnamon and nutmeg.
Alternate adding dry ingredients and sour milk to the mixing bowl.
Stir in 1/2 cup chopped walnuts saving a few to sprinkle on top before baking.
Add batter to oiled muffin tin.
Bake for 18-20 minutes.