1. Preheat your oven to 400 F (200 C). Place a 10-inch cast-iron skillet in the oven while it preheats. Once the oven is hot, carefully remove the hot skillet and add 2 tablespoons of butter or oil, swirling to coat the bottom and sides. Return it to the oven while you mix the batter.

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

3. In a separate medium bowl, whisk together the sourdough discard, buttermilk, melted butter, and eggs until smooth and fully combined.

4. Pour the wet ingredient mixture into the dry ingredient 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. If the batter seems too thick, you can add the 1/4 cup of whole milk, a tablespoon at a time, until it reaches a thick but pourable consistency.

5. Carefully remove the hot, greased cast-iron skillet from the oven. Pour the cornbread batter evenly into the hot skillet. The batter should sizzle slightly as it hits the hot surface.

6. Place the skillet back into the preheated oven and bake for 20-25 minutes, or until the top is golden brown and a wooden skewer or toothpick inserted into the center comes out clean.

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.