Add the cornmeal and flour to a non-reactive. You can use a wood, glass, pottery, or enamel bowl but try to avoid metal bowls.
To that, add the sourdough starter and buttermilk. With a whisk, mix all these ingredients until well combined, cover the bowl, and let the dough sit for at least 4 hours.
Preheat the oven to 350˚F.
Once everything is properly fermented, add the remaining ingredients. I like to add the baking powder and baking soda last, giving the batter only a quick stir.
I like to give this cornbread a "jump-start" by heating up a cast-iron skillet on the stove. Then I add 1-2 TBSP of oil and pour the batter into the hot skillet. When the edges are beginning to bake, transfer the skillet to the oven.
Bake for 35 minutes or until an inserted toothpick comes out clean.