Mix the cornmeal and milk kefir (see other options for other ideas) in a large glass jar until combined. Lightly cover with a cheesecloth or piece of scrap fabric, or coffee filter and allow to sit for 8-10 hours.
After 8-10 hours, the cornmeal should be visibly fermented (air pockets). Dump the jar contents into a large bowl and stir in the remaining ingredients.
Preheat the oven to 375 degrees F. Grease a pie dish or oven-safe baking dish with a little butter. Pour the batter in the baking dish.
Bake for 20-25 minutes or until a toothpick inserted in the center comes out clean and the cornbread starts to brown on the edges.
Notes
Feel free to sub the kefir with buttermilk.
I bake my cornbread in a 9" glass pie dish.. but you could also use a square baking dish or even a muffin tin. If using a muffin tin, you might want to reduce the baking time by 5-10 minutes.