Preheat oven to 425 degrees. Place cast-iron skillet in oven as it is preheating.
Mix dry ingredients in a bowl and set aside. In another bowl mix milk, egg and oil until blended. Combine wet ingredients with dry ingredients and stir until moistened and fairly smooth (about 1 minute.)
Carefully remove hot skillet from oven and grease with butter. Pour batter into skillet and bake 20 to 23 minutes or until toothpick inserted in the center comes out clean.