Combine first 3 ingredients in a 2-cup glass measure. Microwave at high 2 minutes; let stand 2 minutes.
Add butter, stirring until melted. Cool to room temperature.Lightly spoon flour into dry measuring cups, and level with a knife.
Combine flour, 2/3 cup sugar, baking powder, and salt in a large bowl; make a well in center of mixture.
Combine milk mixture and egg, and add to flour mixture, stirring just until moist. Spoon batter into 12 muffin cups coated with cooking spray.
Sprinkle 1 tablespoon sugar evenly over muffins.
Bake at 350 for 20 minutes or until the muffins spring back when lightly touched in center.
Remove the muffins from the pan immediately, and place on a wire rack.Note: Store the remaining muffins in a heavy-duty zip-top plastic bag in the freezer for up to 2 months. Thaw at room temperature.
