Preheat your oven to 425°F (220°C). Place a cast-iron skillet in the oven to heat as the oven warms up.
Mix dry ingredients. In a large bowl, combine the cornmeal, flour, sugar (if using), baking powder, baking soda, and salt.
Add wet ingredients. In a separate bowl, whisk together the buttermilk and egg. Pour the wet mixture into the dry ingredients, stirring until just combined. Then, add the melted butter and mix until smooth.
Bake. Carefully remove the hot skillet from the oven and grease it with a bit of additional butter. Pour the batter into the skillet, spreading it evenly. Bake for 20-25 minutes, or until the top is golden brown and a toothpick inserted into the center comes out clean.
Serve. Let the cornbread cool slightly before slicing. Serve warm, ideally with a pat of butter or a drizzle of honey.