Pat chicken pieces dry with paper towels. If using breasts, slice horizontally to make two thinner cutlets about 1/2 inch thick. In a shallow bowl combine buttermilk and hot sauce. Add chicken, turn to coat, cover and refrigerate for at least 15 minutes and up to 2 hours. This tenderizes and flavors the chicken.

Make the dredge

In a second shallow dish whisk together flour, cornstarch, paprika, garlic powder, onion powder, salt, and black pepper until evenly combined.

Coat the chicken

Remove chicken from buttermilk, letting excess drip off. Dredge each piece thoroughly in the flour mixture, pressing so the coating adheres. Shake off excess and set on a plate. For extra-crispy coating, dip back briefly into a tablespoon of buttermilk then re-dredge in the flour mixture.

Fry the chicken

Pour oil into a heavy skillet (cast-iron preferred) to a depth of about 1/4 inch and heat over medium-high until shimmering and a small pinch of flour sizzles on contact (about 350°F / 175°C). Carefully add chicken pieces (do not overcrowd; fry in batches if needed). Cook 3–4 minutes per side for thighs/cutlets, until golden brown and internal temperature reaches 165°F (74°C). Transfer to a wire rack set over a sheet pan and rest 3–5 minutes.

For the sandwich assembly

Toast the buns

Butter the cut sides of the buns lightly. Heat a skillet over medium heat and toast buttered sides until golden, about 1–2 minutes. Remove from heat.

Prepare sauce and vegetables

In a small bowl mix mayonnaise and mustard (if using). Arrange lettuce leaves, tomato slices, and pickles so they are ready to assemble.

Assemble

Spread mayonnaise mixture on both toasted bun halves. Place a lettuce leaf on the bottom bun, then a piece of fried chicken, 2 tomato slices, and 3 pickle slices. Squeeze a little lemon over the tomato if using and add extra hot sauce to taste. Top with the bun crown.

Serving

Serve the sandwiches hot immediately while the chicken is juicy and the coating is crisp. Accompany with fries, a simple salad, or extra pickles.