Place a 10-inch cast iron skillet in the oven and preheat to 220°C (425°F).
Whisk together cornmeal, flour, baking powder, baking soda, salt, and sugar in a large bowl.
In a separate jug, whisk buttermilk, eggs, and melted butter until smooth.
Pour the wet ingredients into the dry and fold together until just combined. Do not overmix.
Remove the hot skillet from the oven using an oven mitt. Add 1 tablespoon of butter and swirl to coat. Pour in the batter — it should sizzle immediately.
Bake for 18 to 20 minutes until the top is deep golden brown and a toothpick inserted in the center comes out clean.
Rest 5 minutes before slicing and serving.