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 until evenly blended.
03 - Pour vegetable oil into deep fryer or heavy-bottomed pot and heat to 350°F.
04 - Lift chicken from marinade, allowing excess liquid to drip off. Press each piece firmly into flour mixture, turning to coat completely and creating a thick, craggy coating.
05 - Lower coated chicken carefully into hot oil. Fry for 6 to 8 minutes per piece, turning once, until deep golden brown and internal temperature reaches 165°F. Transfer to wire rack or paper towels to drain.
06 - Lightly toast cut sides of burger buns until golden and warm.
07 - Stir together mayonnaise and hot sauce in small bowl until combined, adjusting heat to preference.
08 - Spread spicy mayo on bottom bun. Layer lettuce, fried chicken fillet, tomato slices, and pickles. Crown with top bun and serve immediately.