01 - Place chicken breasts between two sheets of parchment paper and gently pound to even thickness, approximately 1/2 inch thick.
02 - Whisk together buttermilk, salt, black pepper, garlic powder, and paprika in a bowl. Submerge chicken pieces in the mixture, cover, and refrigerate for at least 30 minutes, up to 4 hours for optimal flavor.
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 a depth of 2 inches. Heat to 350°F, maintaining temperature throughout frying.
05 - Remove chicken from marinade, allowing excess liquid to drip off. Press each piece firmly into the flour mixture, ensuring a 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 crispy, ensuring 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 and fully incorporated.
08 - Spread prepared sauce on both halves of each toasted bun. Layer bottom bun with shredded lettuce, followed by fried chicken, pickle slices, and tomato. Place bun top and serve immediately.