Preheat the oven to 400°. Line a 12-cup muffin pan with paper liners. Lightly coat liners with cooking spray.
Stir together flour, sugar, baking powder, and salt in a large bowl; make a well in the center of the mixture.
Mix together egg, milk, and butter; add to the flour mixture, stirring just until moistened. The batter will be lumpy. Spoon batter into the prepared muffin pan, filling cups three-quarters full.
Bake until light golden, about 20 minutes. Remove the muffins from the pan immediately and cool on a wire rack.