Place your 10-inch cast iron skillet in the oven and preheat to 425 degrees F. Let the skillet heat for at least five minutes while you mix the batter. This step is what gives you that crispy bottom crust, so do not skip it.

In a large bowl, whisk together the cornmeal, flour, sugar, baking powder, baking soda, and salt until evenly combined. In a medium bowl, whisk together the buttermilk, beaten egg, and 2 tablespoons of the melted butter.

Pour the wet ingredients into the dry ingredients and stir with a spoon or spatula just until the batter comes together. A few small lumps are fine. Overmixing makes cornbread tough, so stop as soon as you no longer see dry streaks of flour.

Carefully remove the hot skillet from the oven using heavy oven mitts. Add the remaining 2 tablespoons of butter directly to the skillet and swirl it around quickly so it coats the bottom and sides. The butter will sizzle and foam, and that is exactly what you want.

Immediately pour the batter into the hot buttered skillet. It should sizzle when it hits the pan. Smooth the top lightly with your spatula, then return the skillet to the oven.

Bake for 20 to 22 minutes, until the top is golden brown and a toothpick inserted in the center comes out clean. The edges will have pulled away slightly from the sides of the skillet. Let it cool in the pan for five minutes before slicing into wedges and serving.