01 - Place chicken breasts between two sheets of parchment paper and gently pound to even thickness (about 1/4 inch).
02 - In a bowl, combine buttermilk, 1 teaspoon salt, black pepper, garlic powder, and 1 teaspoon paprika. Add chicken and marinate in refrigerator for at least 30 minutes, up to 4 hours.
03 - In a shallow dish, whisk together flour, cornstarch, 1 teaspoon salt, 1 teaspoon paprika, and cayenne pepper until well blended.
04 - Pour vegetable oil into a deep skillet or Dutch oven to depth of 2 inches. Heat to 350°F.
05 - Remove chicken from marinade, shaking off excess liquid. Press each piece firmly into flour mixture, ensuring thick, even coating.
06 - Carefully place chicken in hot oil. Fry 4 to 5 minutes per side until golden brown and internal temperature reaches 165°F. Transfer to wire rack or paper towels to drain.
07 - Whisk together mayonnaise and Dijon mustard in small bowl until smooth.
08 - Spread sauce on both halves of toasted buns. Layer bottom bun with lettuce, fried chicken, pickles, and tomato if desired. Cover with bun top and serve immediately.