Combine first 6 ingredients in a large bowl. Combine egg, buttermilk, and butter; add to dry ingredients, stirring just until moistened.
Heavily grease a 10" cast-iron skillet with shortening. Place in a 425° oven 5 minutes or until very hot (almost smoking). Remove skillet from oven, and spoon batter into hot skillet. Bake at 425° for 25 minutes or until lightly browned. Let cool in pan. Cover loosely, and let stand overnight.