01 - Place chicken breasts between two sheets of parchment paper and gently pound to even thickness of about 0.6 inch.
02 - Whisk together buttermilk, salt, pepper, garlic powder, and paprika in a bowl. Submerge chicken pieces in the mixture, cover, and refrigerate for at least 30 minutes or up to 4 hours.
03 - Combine flour, cornstarch, salt, paprika, and cayenne pepper in a shallow dish, mixing thoroughly.
04 - Pour vegetable oil into a deep skillet or Dutch oven to a depth of 2 inches. Heat to 350°F.
05 - Remove chicken from marinade, allowing excess to drip off. Press each piece firmly into the flour mixture, ensuring complete coverage.
06 - Carefully place coated chicken in hot oil. Fry for 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 - Combine mayonnaise and Dijon mustard in a small bowl, stirring until smooth.
08 - Spread sauce on both halves of toasted buns. Layer lettuce, fried chicken, pickles, and tomato on bottom bun. Place top bun and serve immediately.