- Preheat oven to 400 °F.
- 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.