Place a 10-inch cast iron skillet in the lower portion of your oven (or on the floor if your model allows) and allow to heat with oven to 500 degrees F.
Spin the grits in the carafe of a blender on high for 30 seconds. (If possible with your model, start at low speed and slowly increase speed to high.)
In a large bowl, combine the grits, cornmeal, baking powder, salt, and baking soda. Set aside.
In a separate bowl or large measuring cup, thoroughly combine the buttermilk, oil, and eggs. Add the wet mixture to the dry mixture and whisk smooth.
Open the oven and pull out the skillet just enough to add the butter and batter. Swirl the butter around the bottom of the skillet before quickly pouring in the batter and jiggling to evenly distribute.
Return the skillet to the oven floor (or lowest rack if necessary) and bake for 5 minutes at 500 degrees F. Then move skillet to the middle rack, drop the heat to 425 degrees F and bake another 20 minutes or until the cornbread is golden brown and springs back when touched. An instant-read thermometer should read 200-205 degrees F at the center.
Remove from the oven and cool cornbread in pan for 5 minutes before inverting onto a rack to cool completely. Consume hot with butter and a glass of cold buttermilk on the side.