Preheat oven to 400°.
Lightly spoon flour into dry measuring cups; level with a knife. Combine flour, 1/2 cup sugar, and next 6 ingredients in a large bowl; stir well with a whisk. Make a well in center of mixture.
Combine buttermilk, butter, and egg; stir well with a whisk. Add to flour mixture, stirring just until moist.
Spoon batter into 15 muffin cups coated with cooking spray; sprinkle evenly with remaining sugar.
Bake at 400° for 15 minutes or until muffins spring back when touched lightly in center.
Remove muffins from pan immediately; cool on a wire rack.