01 - Whisk together buttermilk, hot sauce, garlic powder, onion powder, salt, and pepper in a large bowl. Submerge chicken thighs completely, cover, and refrigerate for minimum 2 hours or overnight for optimal flavor penetration.
02 - Combine flour, cornstarch, paprika, baking powder, cayenne, salt, and black pepper in a shallow dish, whisking thoroughly to distribute spices evenly.
03 - Pour vegetable oil into deep fryer or heavy-bottomed pot. Bring temperature to 350°F, maintaining steady heat throughout frying process.
04 - Lift chicken from marinade, allowing excess liquid to drip off. Press each piece firmly into flour mixture, turning to coat completely and creating thick, craggy exterior for maximum crunch.
05 - Lower chicken pieces carefully into hot oil using tongs. Fry 6 to 8 minutes, flipping once halfway through, until deep golden brown and internal temperature reaches 165°F. Transfer to wire rack to drain.
06 - Lightly toast cut sides of brioche buns until golden and warm to the touch.
07 - Blend mayonnaise with hot sauce if adding spicy kick to sandwich.
08 - Spread mayonnaise mixture on toasted bottom bun. Layer lettuce leaf, fried chicken thigh, tomato slice, and pickle chips. Crown with top bun and serve immediately while chicken remains hot and crispy.