Preheat oven to 425 degrees. Whisk together cornmeal, flour, sugar, baking powder, baking soda, and salt in a large bowl; set aside. Whisk together egg and buttermilk; stir into flour mixture.
Melt butter in a 10-inch cast-iron skillet or a 2-quart baking dish in oven. Remove skillet; swirl to coat bottom. Pour in batter. Bake until a tester comes out clean, 20 to 25 minutes. Cut into wedges.