01 - Place chicken breasts between two sheets of parchment paper. Gently pound to even thickness of approximately 0.6 inch using a meat mallet.
02 - Whisk together buttermilk, 1 teaspoon salt, black pepper, garlic powder, and paprika in a bowl. Submerge chicken pieces in the mixture, cover, and refrigerate for 30 minutes to 4 hours.
03 - Combine flour, cornstarch, 1 teaspoon salt, paprika, and cayenne pepper in a shallow dish. Mix thoroughly to distribute spices evenly.
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, allowing excess to drip off. Press each piece firmly into flour mixture, ensuring complete coverage. Shake off loose coating.
06 - Carefully place coated chicken into hot oil. Fry for 4 to 5 minutes per side until golden brown and internal temperature reaches 165°F. Transfer to wire rack to drain.
07 - Combine mayonnaise and Dijon mustard in a small bowl. Stir until smooth and fully incorporated.
08 - Spread sauce evenly on cut sides of toasted buns. Place lettuce on bottom bun, add fried chicken, arrange pickles and tomato slices. Top with bun crown and serve immediately.