Place chicken in a gallon-sized plastic zipper bag and add Charleston Mix, salt, and sugar. Refrigerate overnight.
Prepare the dredge by adding the flour to a large bowl or baking dish and whisking together with salt and pepper to taste. In another large bowl or baking dish, combine buttermilk with a generous pinch of salt.
Heat the oil to 350°F over medium heat in a large, heavy-bottomed pot or Dutch oven.
Drain brined chicken well and discard brine. Place chicken, a few pieces at a time, into the flour and toss to coat. Shake excess flour from the chicken, coat in buttermilk, and then return to the flour to toss and coat again. Let the chicken rest on a wire rack while you finish dredging the other pieces.
Fry the chicken 3 pieces at a time for 15-20 minutes, or until an instant-read thermometer inserted into the thickest part of the meat reaches 157°F. Remove chicken to a paper towel-lined plate and sprinkle with salt while still hot.