1. Preheat oven to 425 degrees F. Place an oven-proof 10-inch cast iron skillet in the oven.
2. In a large bowl, combine cornmeal, flour, sugar, baking powder and salt.
3. In a large glass measuring cup or another bowl, whisk together buttermilk, egg and 3 tablespoons butter. Pour mixture over dry ingredients and stir using a rubber spatula just until moist.
4. Remove skillet from the oven and add remaining 3 tablespoons until heated through, about 1-2 minutes.
5. Working carefully, immediately scoop the batter into the hot skillet. Place into oven and bake for 25-30 minutes, or until a tester inserted in the center comes out clean.
6. Serve warm right in the cast iron skillet to keep warm and for presentation purposes.