01 - Combine chicken, pickle brine, and hot sauce if using in a bowl or resealable bag. Marinate in 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 - Whisk together flour, cornstarch, paprika, garlic powder, onion powder, cayenne pepper, kosher salt, and black pepper in a shallow dish.
04 - Remove chicken from brine, allowing excess to drip off. Dredge thoroughly in flour mixture to coat completely on all sides.
05 - Fry chicken pieces in batches for 5 to 7 minutes per side until golden brown and cooked through, reaching internal temperature of 165°F. Drain on wire rack or paper towels.
06 - Mix mayonnaise and Dijon mustard in a small bowl until combined.
07 - Lightly toast buns and spread mayo-mustard sauce on both cut sides.
08 - Layer fried chicken, shredded lettuce, and pickle slices on bottom buns. Top with remaining bun halves and serve immediately.