Preheat oven to 375°F and place your cast iron skillet or baking dish in the oven to get nice and hot. Mix milk and vinegar and let sit if not using buttermilk.
Mix flour, cornmeal, sugar, baking soda, baking powder, and salt in separate bowl.
Mix buttermilk and butter then add eggs. Make sure it is combined well and the eggs are whisked and broken up nicely.
Add wet ingredients to the dry ingredients. Fold gently, until the wet and dry ingredients are just combined. If you over-mix, your batter won't be able to rise and your cornbread will be flat.
Once mixed, carefully remove your hot skillet from the oven. Coat the skillet with butter. Pour your batter into the hot skillet and place the skillet back in the oven. Bake for about 30 minutes. Cornbread is done once the edges pull away from the pan. If you poke the cornbread, it should not sink in. Cornbread is pretty stiff when it is done, but it has a slight spring when you touch it.
Keto version- 1 1/2 cups almond flour, 1 tsp baking powder, 1/2 cup sweetener, 4 eggs, 4 Tbsp butter (melted) plus 1 Tbsp for skillet. Bake 350 for 20 minutes.