Preheat oven to 425°F. Place an oven-proof 10-inch cast iron skillet in the oven.
In a large bowl, combine corn flour, flour, sugar, baking powder, and salt.
In a large glass measuring cup or other bowl, whisk together buttermilk, egg and three tablespoons butter. Pour mixture over dry ingredients and stir using a rubber spatula just until moist
Remove skillet from the oven and add remaining 3 tablespoons butter until heated through, about one to two minutes.
Working carefully, immediately scooped the batter into the hot skillet. Place into oven and bake for 20-25 minutes, or until a tester inserted in the center comes out clean. Let cool 5 minutes.