Preheat oven to 350F. Line 12 muffin cups with foil liners (I just use the cheap paper ones). Mix flour, sugar, baking powder, and salt in a large bowl. Mix mashed bananas, egg, melted butter, and milk in a medium bowl (I heat the milk a little in the microwave as well to keep the butter from clumping, as it will if you add cold milk). Stir banana mixture into dry ingredients just until blended. Stir in chocolate chips and nuts.
Divide batter among prepared muffin cups, filling each about 3/4 full. Bake until tops are pale golden and tester inserted into center comes out with no crumbs, about 32 minutes. Transfer muffins to rack to cool.
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