Preheat oven to 425°F. Line 12-count muffin pan with paper liners or coat with cooking spray.
In a small bowl, whisk the flour, baking powder, baking soda, salt, cinnamon, and nutmeg together. Set aside.
In a medium bowl, mash bananas. Whisk in the melted butter, eggs, vanilla and milk or yogurt.
Add the dry ingredients and use a spatula to gently mix just until combined. Fold in walnuts and chocolate chips.
Spoon the batter into muffin pan, filling all the way to the top. Bake for 5 minutes at 425. Then, with the muffins in the oven, reduce the temperature to 350°. Bake for an additional 15-16 minutes, until a toothpick comes out clean.
Allow the muffins to cool for a couple minutes in the pan, then transfer to a cooling rack.