Whisk buttermilk, 1 tbsp hot sauce, ½ tsp salt, and pepper in a bowl. Add chicken thighs, coat well, cover, and refrigerate for at least 30 minutes or up to 4 hours.

Stir mayonnaise with 1 to 2 tbsp hot sauce and a pinch of salt until smooth; chill until ready to use.

Whisk flour, cornstarch, paprika, garlic powder, onion powder, cayenne, and ½ tsp salt in a shallow dish. Remove thighs from marinade, let excess drip off, and press firmly into the flour mixture to coat completely. Shake off loose flour and rest on a wire rack for 5 minutes.

Heat 1½ to 2 inches of vegetable oil in a heavy pot to 350°F (175°C). Fry thighs in batches of two for about 4 minutes per side, until deep golden and an instant-read thermometer inserted into the thickest part reaches 165°F (74°C). Drain on a rack set over paper towels.

Toast buns cut side down until golden, about 1 minute. Spread spicy mayo on both halves, then layer lettuce, a fried thigh, tomato slices, and pickles. Serve hot.