Preheat oven to 350°F. Spray a standard muffin tin with non-stick cooking spray and set aside.
Add the mashed bananas, melted butter, and milk to a large mixing bowl. Mix to combine.
Beat in the egg and vanilla, and brown sugar.
Add in the remaining dry ingredients and fold in until just mixed. Then, fold in the walnuts.
Fill muffin pan cups to about 4/5 full. Top with additional walnuts, if desired.
Place in the pre-heated oven and bake for 18-20 minutes, or until toothpick inserted in the center of a muffin comes out clean. Remove from oven and cool for at least 5-10 minutes before serving.