01 - Place chicken breasts between two sheets of parchment paper and gently pound to even thickness, about 0.6 inch.
02 - In a bowl, combine buttermilk, salt, pepper, garlic powder, and paprika. Add chicken, cover, and refrigerate for at least 30 minutes, up to 4 hours for optimal flavor.
03 - In a shallow dish, whisk together flour, cornstarch, salt, paprika, and cayenne pepper until well blended.
04 - Pour vegetable oil into a deep skillet or Dutch oven to a depth of about 2 inches. Heat to 350°F.
05 - Remove chicken from marinade, shaking off excess liquid. Dredge each piece in the flour mixture, pressing firmly to ensure thick, even coating.
06 - Carefully place coated chicken in hot oil. Fry for 4 to 5 minutes per side until golden brown and cooked through, reaching an internal temperature of 165°F. Transfer to wire rack or paper towels to drain.
07 - Whisk together mayonnaise and Dijon mustard in a small bowl until smooth and well combined.
08 - Spread sauce on both halves of each toasted bun. On bottom bun, layer shredded lettuce, fried chicken, pickles, and tomato slices if using. Crown with bun top and serve immediately.