Mash bananas in mixer on low to medium speed until well mashed. Add in the remaining (except chocolate chips and walnuts) ingredients one at a time while mixing on low speed. Add in ½ of the chocolate chips.
Spray muffin pan with cooking spray. I used two jumbo muffin pans, and the recipe made 12 jumbo muffins.
Divide muffin batter between muffin cups. Sprinkle remaining chocolate chips and the chopped walnuts over the top of the unbaked muffins.
Bake on 375 for 15-20 minutes or until a toothpick stuck in the center of a muffin comes out clean.