Preheat oven to 350°F. Place two tablespoons of the butter in a 9” cast iron skillet and set aside.
Melt the remaining two tablespoons of butter in a microwave safe bowl.
In a medium mixing bowl, stir together the buttermilk, vegetable oil, eggs, and the two tablespoons of melted butter.
In a separate bowl, whisk together cornmeal, flour, sugar, baking powder, and salt.
Make a well in the center of the dry ingredients. Pour the wet ingredients into the well and stir the ingredients together, just wetting the dry ingredients - the batter will be lumpy.
On the stovetop, heat the butter in the cast iron skillet until melted. Pour in the cornbread batter. As the melted butter comes up the sides of the batter, gently use a spoon or spatula to spread it over the top of the batter.
Place the skillet in the oven and bake for about 25 minutes or until a toothpick inserted in the center of the cornbread comes out with a few crumbs. The center should be just set.
Serve warm with butter.
Notes
As a buttermilk substitute, mix 1 tablespoon of lemon juice with 1.5 cups of whole milk. Stir and let it side for 5-10 minutes before using.
Instead of a skillet, you can use an 8x8 baking dish. Use about 2 tablespoons of butter to grease the dish, and add the remaining butter to the batter.
A hot cast-iron skillet keeps heat for a long time. If you are not serving the cornbread immediately, transfer to another dish to rest so it doesn't overcook.
To make cornbread muffins, follow the same instructions except stir all of the butter into the batter; no need to add it to cupcake liners. If you do not want to use cupcake liners, then you can divide the melted butter between the muffin tins. Bake for 20 minutes or until they are done.