For the Buttermilk Brine:
1 cup buttermilk
1 large egg
2 tablespoons of the KFC spice mix
For Finishing the Chicken:
1 whole chicken, cut into 10 pieces
1 1/2 cups all-purpose flour
1/2 cup cornstarch
1 teaspoon baking powder
3 tablespoons of the KFC spice mix
Fry Oil
Whisk the buttermilk, egg and 2 tablespoons of the KFC spice mixture in a large bowl. Add the chicken pieces and toss and turn to coat. Transfer the contents of the bowl to a gallon-sized zipper-lock freezer bag and refrigerate for at least 4 hours, and up to overnight, flipping the bag occasionally to redistribute the contents and coat the chicken evenly.
Whisk together the flour, cornstarch, baking powder, and 2 tablespoons of the KFC spice mix in a large bowl. Add 3 tablespoons of the marinade from the zipper-lock bag and work it into the flour with your fingertips. Remove one piece of chicken from the bag, allowing excess buttermilk to drip off, drop the chicken into the flour mixture, and toss to coat. Continue adding chicken pieces to the flour mixture one at a time until they are all in the bowl. Toss the chicken until every piece is thoroughly coated, pressing with your hands to get the flour to adhere in a thick layer.
Heat the oil to 175°C in a 12-inch straight-sided cast-iron chicken fryer or a large wok over medium-high heat. Adjust the heat as necessary to maintain the temperature, being careful not to let the fat get any hotter.
Once all the chicken pieces are coated, place skin side down in the hot oil. The temperature should drop to 163°C; adjust the heat to maintain this temperature for the duration of the cooking. Fry the chicken until it’s a deep golden brown on the first side, about 6 minutes; do not move the chicken or start checking for doneness until it has fried for at least 3 minutes, or you may knock off the coating. Carefully flip the chicken pieces with tongs and cook until the second side is golden brown, about 4 minutes longer. You want an internal temperature on the dark meat to be around 80°C and white meat to be around 74°C.
Transfer chicken to a wire rack set in a rimmed baking sheet, and let cool. When ready to serve, reheat the oil to 190°C. Add the chicken pieces and cook, flipping them once halfway through cooking, until completely crisp, about 5 minutes. Transfer to a wire rack set on a rimmed baking sheet to drain