Place the chicken pieces in a bowl. Pour the buttermilk and hot sauce over the chicken. Cover and refrigerate for at least 4 hours, or preferably overnight.
In a shallow dish, whisk together the flour, cornstarch, salt, pepper, paprika, and garlic powder for the dredge.
Remove the chicken from the buttermilk, letting excess drip off, but do not pat dry. Dredge each piece thoroughly in the flour mixture, pressing the coating onto the chicken. For extra crispiness, dip the floured chicken back into the remaining buttermilk mixture briefly, then dredge again in the flour mixture, ensuring a thick, craggy coating.
Heat about 2 inches of vegetable oil in a deep skillet or Dutch oven to 325°F (160°C).
Carefully place 2 chicken pieces into the hot oil, ensuring you do not overcrowd the pan. Fry for 6 to 8 minutes per side, until the internal temperature reaches 165°F (74°C) and the crust is deep golden brown and crisp. Adjust heat as needed to maintain oil temperature.
Remove the chicken and place it on a wire rack set over a baking sheet to drain excess oil. Sprinkle lightly with salt immediately after removing from the oil.
While the chicken rests, prepare the slaw. In a medium bowl, whisk together the mayonnaise, buttermilk, apple cider vinegar, sugar, celery salt, and pepper until smooth.
Add the shredded cabbage mix to the dressing and toss until everything is evenly coated. Refrigerate for at least 30 minutes before serving to allow flavors to meld.
Lightly toast the brioche buns. Place a crispy chicken breast on the bottom bun. Top with several pickle chips and a generous scoop of the creamy slaw. Place the top bun on the sandwich.
Notes
For the crispiest chicken, use a 50/50 mix of all-purpose flour and cornstarch in your dry dredge.
Maintain the oil temperature strictly between 325°F and 335°F for the best texture and to prevent the crust from burning before the chicken cooks through.
If you prefer a spicy sandwich, add 1 teaspoon of cayenne pepper to the flour dredge.