Preheat oven to 425°.
Lightly spoon flour into a dry measuring cup, and level with a knife. Combine flour and next 5 ingredients (flour through baking soda) in a large bowl, and make a well in center of mixture. Combine buttermilk and next 4 ingredients (buttermilk through egg) in a bowl; add to flour mixture. Stir just until moist. Spoon batter into an 8-inch square baking pan coated with cooking spray. Bake at 425° for 20 minutes or until a wooden pick inserted in center comes out clean. Let stand 10 minutes.