Heat an 8-inch cast iron skillet in the oven while you preheat the oven to 425°F.
Meanwhile, in a large bowl, whisk together the buttermilk, egg, and oil.
Add the cornmeal mix and sugar; stir just until combined. Do not overmix – the batter will be lumpy, which is fine.
Once the oven comes to temperature, carefully remove the skillet. Place the butter in the skillet; use a knife or brush to spread the butter on the bottom and sides of the pan until it melts.
Pour the batter into the buttered skillet.
Bake for about 25 minutes, or until a toothpick inserted in the center comes out clean.
Notes
Make sure that you’re using self-rising cornmeal mix — not plain cornmeal. The self-rising mix includes some flour, salt, and leavening agents in addition to the cornmeal.
Always preheat your cast iron skillet for cornbread. The hot skillet creates a crisp, sturdy crust on the cornbread, which is a flavorful, delicious contrast to the tender, delicate crumb.
Use full-fat buttermilk rather than a lower-fat alternative. The fat in the liquid adds richness to the batter and helps to keep the cornbread moist.
Let the batter rest for 5-10 minutes before baking, if you have the time. This allows the leavening agents to activate and incorporate more air.
Don’t over-mix the batter or it will become too dense, and you’ll end up with dry cornbread.
To check if the cornbread is done, insert a toothpick into the center and make sure it comes out clean. Don’t cook for too long, or it can dry out.