Preheat the oven to 450 degrees F.
Pour 2 tablespoons of the oil into a well-seasoned 9-inch cast-iron skillet (see Cook's Note) and place over medium-high heat. Pour the cornmeal into a large mixing bowl. Make a well in the cornmeal and add the remaining 2 tablespoons oil. With a fork, stir in enough of the buttermilk to make a batter that is thick but can be easily poured into the hot skillet. You may not need all 3 cups.
Carefully pour the batter into the skillet. The oil will come up around the edges. Use the back of a spoon to smooth this over the top of the batter. Continue to heat on the stovetop for 1 minute, then transfer the skillet to the oven and bake the cornbread until browned on top, about 20 minutes. Immediately turn the cornbread out onto a cooling rack to keep the crust crisp.
Season a new cast-iron skillet by coating the inside with vegetable shortening (not butter) and heating the pan in a very hot oven (450 degrees F) for 1 hour. Cool the skillet and wipe off any excess oil before using. To care for a seasoned skillet, wipe the interior after each use. Do not soak in any soapy water or put in the dishwasher.
Adapted from "Georgia Cooking in an Oklahoma Kitchen" by Trisha Yearwood © Clarkson Potter 2008. Provided courtesy of Trisha Yearwood. All rights reserved.