01 - Place chicken breasts between two sheets of parchment paper and gently pound to even thickness, approximately 1/4 inch thick.
02 - Whisk together buttermilk, salt, black pepper, garlic powder, and paprika in a bowl. Submerge chicken pieces completely, cover, and refrigerate for 30 minutes to 4 hours.
03 - Combine flour, cornstarch, salt, paprika, and cayenne pepper in a shallow dish, mixing 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, maintaining temperature throughout frying process.
05 - Remove chicken from marinade, allowing excess to drip off. Press each piece firmly into flour mixture, ensuring thick, even coating on all sides.
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 to drain.
07 - Combine mayonnaise and Dijon mustard in small bowl, whisking until smooth and fully incorporated.
08 - Spread sauce on both halves of toasted buns. Layer bottom bun with lettuce, fried chicken, pickles, and tomato. Place top bun and serve immediately.