Preheat oven to 400. Spray 12 muffin cups with nonstick spray. In medium mixing bowl combine flour, milk powder, sugar, baking powder, baking soda and nutmeg; stir to combine and set aside. In small bowl, combine buttermilk, egg, oil, and butter and stir until blended; stir in dry ingredients. Add banana, raisins, and walnuts and stir to combine (mixture will be lumpy). Fill each baking cup with an equal amount of batter and bake for 15 minutes (until golden brown and a toothpick inserted in center comes out dry). Transfer muffins from pan to wire rack and cool.