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