Combine first 4 ingredients in a large bowl; make a well in center of mixture.
Stir soda into buttermilk. Combine buttermilk mixture, egg, sugar, and oil; add to dry ingredients, stirring just until moistened. Place butter in a 9" or 10" cast-iron skillet coated with cooking spray. Place over medium-high heat until butter melts. Pour batter into prepared skillet. Bake at 375° for 25 to 30 minutes or until golden.