Preheat oven to 475˚F.

Put the bacon grease, lard, or shortening in a 12" cast iron skillet and heat in the oven until the oil shimmers and almost smokes; this is what makes the crispy crust.

Mix cornmeal, baking soda, baking powder, and salt thoroughly. Whisk egg into buttermilk and stir into cornmeal mixture. Pour into hot skillet and bake for 20 minutes, until lightly browned. 

Run a spatula around the edges of the skillet, sliding it under the corn bread, and invert onto a platter or cutting board. Cut into wedges and serve at once.