Preheat the oven to 400°F. Grease a 10-inch cast iron skillet with oil, bacon grease, or cooking spray and place it in the oven to preheat. A square baking dish will also work.
Add the cornmeal, flour, baking powder, baking soda, sugar, and salt to a mixing bowl and whisk to combine.
Stir together the buttermilk and melted butter, then add the egg and whisk to combine.
Combine the wet ingredients and dry ingredients and stir to combine to form a thick batter.
Carefully remove the skillet from the oven and scrape the batter into the pan into an even thickness and smooth out the top. Return the skillet to the oven to bake for 20 to 25 minutes until a toothpick inserted into the center comes out clean and the top is golden brown. Let cool for 10 mintutes before serving.