01 - Place chicken breasts between two sheets of parchment paper. Gently pound to an even thickness of about 1/2 inch using a meat mallet or rolling pin.
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. Mix thoroughly to ensure even distribution of spices.
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 to drip off. Press each piece firmly into the flour mixture, ensuring a thick, even coating on all sides. Shake off any loose flour.
06 - Carefully place coated chicken into the hot oil. Fry for 4 to 5 minutes per side until golden brown and crispy. Internal temperature should reach 165°F. Transfer to a wire rack or paper towels to drain.
07 - Blend mayonnaise and Dijon mustard in a small bowl until smooth and well combined.
08 - Spread the prepared sauce on both halves of each toasted bun. Layer shredded lettuce on the bottom bun, followed by the crispy fried chicken, pickle slices, and tomato. Place the top bun and serve immediately while hot.