Preheat the oven to 425 degrees. Combine the dry ingredients in a large bowl. Combine the buttermilk, egg, and oil and add to the dry ingredients, stirring just until the dry ingredients are moistened.

Pour the batter into an 8- by 8-inch pan or cast-iron skillet. (If using cast-iron, preheat the skillet in the oven for 3 minutes before adding the batter.) Bake for 20 minutes.