Mix cornmeal, flour, oil, milk, and sourdough starter in a large mixing bowl.
Cover and allow to ferment for 6-12 hours.
Preheat oven to 425 degrees F with 10 inch cast iron skillet inside.
Add the salt, egg, baking soda, and baking powder. Mix well.
Carefully remove the hot cast iron skillet and add 2 tablespoons of butter.
Pour in the batter.
Bake for 12-15 minutes or until a toothpick comes out clean.