Preheat oven to 425°F. Place butter in a 9 inch cast iron skillet and place into oven until melted, then remove from oven. Set aside.
In a medium bowl, whisk together cornmeal, flour, sugar, baking powder, baking soda, and salt. Set aside.
In a large bowl, stir together the egg and buttermilk. Stir in flour mixture until just combined. Pour mixture into buttered skillet.
Bake from 20-22 minutes, or until bread is golden brown and toothpick inserted in middle comes out clean. Let cool for 10 minutes in skillet before serving.