Preheat oven to 400° F. Place a 10-inch cast iron skillet in the hot oven to heat for 10 minutes (doing this gives the cornbread a crunchy exterior). Have your oven mitt at the ready so you can use it when you remove the pan from the oven to add the batter—please be careful.
Whisk together the dry ingredients in a medium-sized bowl. In another bowl, whisk together the buttermilk, soda water, honey, melted butter, and eggs. Pour liquid ingredients over dry ingredients and mix just until moistened. Stir in the corn kernels or any other optional ingredients.
Remove hot pan from the oven (oven mitt!) and swirl 1 tablespoon butter around in the pan to coat the bottom and up the sides a bit. Pour batter into center of pan, return to oven and bake for 18–20 minutes. Cornbread is done when it is lightly browned on top and begins to pull away from the sides of the pan. (If you're making the fully loaded version, bake about 30 minutes and check doneness by using a knife to poke the center. It will come out clean when the cornbread is done.)