Add the oil to a 10" cast iron skillet, swilling it around to coat as evenly as possible.
Place the skillet in a cold oven, and set it to preheat to 400°.
To a large mixing bowl, add the dry ingredients: flour, cornmeal, sugar, baking powder, baking soda, and salt. Whisk them together to evenly combine.
To the flour mixture, add the buttermilk, melted butter, and eggs. Whisk them in until the mixture's evenly combined.
To the mixing bowl, add a cup of cheese, corn, and the jalapeno. Use a spatula to evenly incorporate.
Using something like an oven safe potholder, remove the hot skillet from the preheated oven. Swill the oil around in the skillet once again.
Pour the cornbread batter into the preheated skillet, scraping out of the bowl with a spatula as needed.
Sprinkle the remaining cheese evenly out over top, and add a few jalapeno slices to the top (if desired), for garnish.
Bake the cornbread from 20-25 minutes, or until a skewer inserted in the center comes out clean and the cheese has formed a golden brown crust on top.
Carefully remove the cornbread skillet from the oven, and let it rest for 10 full minutes.
Run a butter knife gently around the edges of the cornbread to release it from the pan. Then slice, and serve.