How to Make It
Place a 10-inch cast-iron skillet in oven. Preheat oven to 450° (leave pan in oven as it preheats).
Weigh or lightly spoon cornmeal and flour into dry measuring cups; level with a knife. Combine cornmeal, flour, baking powder, salt, and baking soda in a large bowl, stirring with a whisk.
Combine buttermilk, oil, and eggs, stirring with a whisk. Add to cornmeal mixture; stir just until batter is combined.
Cut butter into pieces. Carefully place butter in hot skillet in oven; bake at 450° for 1 to 2 minutes or until butter melts and begins to brown. Carefully remove pan from oven, and pour butter into batter, stirring just until combined. Immediately pour batter into pan. Bake at 450° for 20 minutes or until browned on edges and lightly browned on top. Cool 5 minutes before slicing; serve warm.