Place caste iron skillet in oven and preheat oven to 450°.
Combine cornmeal, flour, salt and baking powder in a large bowl.
Measure buttermilk and milk in a large glass measuring cup. Add the beaten egg. Stir in the baking soda.
Stir the buttermilk mixture into the cornmeal mixture.
Melt 1/4 cup of butter and slowly add to batter and stir until just combined.
Remove skillet from oven and place on stovetop over medium-high heat. Melt remaining 2 tbsps of butter in skillet and add batter. It should sizzle. Cook for a minute then place in oven and bake for 25 minutes or until golden brown.