1. 1. Preheat your oven to 400 F (200 C). Place a 10-inch cast iron skillet in the oven while it preheats. This step is crucial for achieving that wonderfully crispy bottom crust.

2. 2. In a large mixing bowl, whisk together the cornmeal, all-purpose flour, optional granulated sugar, baking powder, baking soda, and salt. Make sure there are no lumps and everything is well combined.

3. 3. In a separate medium bowl, whisk together the sourdough discard, buttermilk, milk, and egg until smooth.

4. 4. Pour the wet ingredients into the dry ingredients. Add the 1/4 cup of melted butter to the mixture. Stir gently with a spoon or spatula until just combined. Be careful not to overmix; a few small lumps are perfectly fine and will result in a more tender cornbread. Overmixing can lead to a tough texture.

5. 5. Carefully remove the hot cast iron skillet from the oven using oven mitts. Add a tablespoon of butter to the hot skillet and swirl it around to coat the bottom and sides thoroughly. The butter should melt and sizzle immediately.

6. 6. Pour the cornbread batter evenly into the hot, buttered skillet. Return the skillet to the preheated oven and bake for 25-30 minutes, or until the top is golden brown and a wooden skewer or toothpick inserted into the center comes out clean.

7. 7. Once baked, carefully remove the skillet from the oven. Let the cornbread cool in the skillet for about 5-10 minutes before slicing and serving. This allows it to set and makes it easier to cut.