Preheat your grill for medium-high heat. If using your oven, preheat to 400 degrees F.
Add the oil to the cast iron pan and place it into your grill or oven to preheat.
Gather your ingredients. Combine your dry ingredients in one bowl, and combine your wet ingredients (melted butter, eggs, milk, and buttermilk) in the other. Slowly mix wet ingredients into the dry ingredients, and gently combine until smooth. Set aside.
Using heat-resistant gloves, remove the skillet from your grill (or oven), and place it onto a heat safe space. Reduce your cooker temperature to medium heat (or 350 degrees F. for your oven).
Move the skillet around to make sure the oil has coated the pan. Use a kitchen brush if needed.
Pour cornbread batter into the skillet and place back into the cooker or oven for 22-25 minutes--or until a toothpick inserted into the center comes out clean and the surface color is a light golden brown.
Remove skillet from heat and cool for at least 15-20 minutes before digging in.