Start by preparing the brine. In a large bowl, whisk together the buttermilk, pickle juice, mustard powder, smoked paprika, hot sauce, salt, pepper, garlic powder, onion powder, celery seed, cayenne, and any optional seasoning blend. Submerge the chicken pieces, cover, and refrigerate for at least 8 hours or overnight.
While the chicken brines, make the special sauce. Stir together the mayonnaise, BBQ sauce, ketchup, mustard, vinegar, salt, and pepper until smooth. Thin with a splash of water if you’d like a drizzle-friendly texture. Store in the fridge until ready to use.
When ready to fry, remove the chicken from the brine and let the excess drip off. In a large shallow dish or sheet pan, whisk together the flour, salt, pepper, smoked paprika, garlic powder, onion powder, mustard powder, celery seed, and cayenne. Dredge each piece of chicken in the flour mixture, pressing firmly so the coating adheres well.
Heat a few inches of oil in a heavy-bottomed pot or cast iron skillet to 350°F. Fry the chicken in batches, turning occasionally, until golden brown and crisp, about 5–8 minutes per side depending on thickness. The internal temperature should read 165°F. Drain on a wire rack or paper towels. If needed, keep warm in a 180°F oven on a rack for up to 30 minutes.
To assemble, toast the buns if desired, then spread the special sauce on both sides. Add a piece of fried chicken, followed by lettuce, tomato, and pickle slices. Top with the remaining bun, press gently, and serve immediately while hot and crispy.