Preheat oven to 425°. Pour oil into a deep 8-inch cast-iron skillet. Place pan in oven until oil is very hot, about 8 minutes.In a large bowl, whisk together cornmeal, flour, baking powder, and salt. In a small bowl, whisk together buttermilk, melted butter, and egg. Make a well in center of dry ingredients. Add buttermilk mixture; stir until combined. Carefully pour batter into hot oil.Bake until golden brown and a wooden pick inserted in center comes out clean, about 25 minutes.Follow previous directions using a 10-inch skillet; bake until golden brown and a wooden pick inserted in center comes out clean, about 27 minutes.Follow previous directions using a 12-inch skillet; bake until golden brown and a wooden pick inserted in center comes out clean, about 35 minutes.