01 - Combine chicken, pickle brine, and hot sauce (if using) in a bowl or resealable bag. Marinate in the refrigerator for at least 1 hour, up to overnight.
02 - Heat vegetable oil in a heavy-bottomed pot or deep skillet to 350°F.
03 - In a shallow dish, whisk together flour, cornstarch, paprika, garlic powder, onion powder, cayenne, salt, and pepper until well combined.
04 - Remove chicken from brine, allowing excess liquid to drip off. Dredge thoroughly in the flour mixture, pressing to coat on all sides.
05 - Fry chicken pieces in batches for 5–7 minutes per side until golden brown and cooked through (internal temperature reaches 165°F). Drain on a wire rack or paper towels.
06 - Mix mayonnaise and Dijon mustard in a small bowl until smooth.
07 - Lightly toast the buns. Spread the mayo-mustard sauce on both sides.
08 - Layer fried chicken, shredded lettuce, and pickle slices onto the bottom buns. Top with remaining bun halves and serve immediately.