01 - Place chicken breasts between two sheets of parchment paper and gently pound to even thickness, about 1/2 to 3/4 inch thick.
02 - In a mixing bowl, combine buttermilk, 1 teaspoon salt, black pepper, garlic powder, and 1 teaspoon paprika. Add the chicken pieces, turning to coat. Cover and refrigerate for at least 30 minutes, up to 4 hours for optimal flavor.
03 - In a shallow dish, whisk together flour, cornstarch, 1 teaspoon salt, 1 teaspoon paprika, and cayenne pepper until evenly combined.
04 - Pour vegetable oil into a deep skillet or Dutch oven to a depth of about 2 inches. Heat over medium-high heat until the temperature reaches 350°F.
05 - Remove chicken from the marinade, allowing excess liquid to drip off. Dredge each piece in the flour mixture, pressing firmly to ensure a thick, even coating. Shake off any excess flour.
06 - Carefully place coated chicken in the 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 a wire rack or paper towels to drain.
07 - In a small bowl, whisk together mayonnaise and Dijon mustard until smooth and well combined.
08 - Spread the prepared sauce on both halves of each toasted bun. On the bottom bun, layer shredded lettuce, followed by the fried chicken breast, pickle slices, and tomato if using. Place the bun top on top and serve immediately.