Melt butter in an 8-inch cast-iron skillet in a 375° oven for 5 minutes.
Combine cornmeal and next 5 ingredients; make a well in center of mixture.
Combine buttermilk, egg, and oil; add to dry ingredients, stirring just until dry ingredients are moistened.
Remove skillet from oven. Pour batter into skillet. Bake at 375° for 25 minutes or until golden.