01 - Combine chicken, pickle brine, and hot sauce in a resealable bag. Marinate in the refrigerator for at least 1 hour, up to overnight.
02 - Pour vegetable oil into a heavy-bottomed pot or deep skillet. Heat 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. Press firmly into the flour mixture, coating all sides thoroughly.
05 - Fry chicken pieces in batches for 5-7 minutes per side until golden brown and internal temperature reaches 165°F. Drain on wire rack.
06 - Combine mayonnaise and Dijon mustard in a small bowl, mixing until smooth.
07 - Lightly toast the brioche buns until golden.
08 - Spread mayo-mustard sauce on both bun halves. Layer with fried chicken, shredded lettuce, and pickle slices. Top with remaining bun and serve immediately.