Place chicken breasts between two long sheets of plastic wrap. Pound to even 1/2-inch thickness. Slice down to slightly larger than your bun size (use any extra chicken pieces as nuggets).
In a large bowl, whisk together buttermilk, egg, and salt. Remove 2 tablespoons of the mixture and set aside. Add chicken to the bowl and coat well. Cover and marinate in the fridge for at least 1 hour up to 24 hours.
In a separate bowl, whisk together the flour, cornstarch, and all listed seasonings. Add the reserved 2 tablespoons of marinade to the breading and mix with your fingers to create small clumps for extra crispiness.
Add oil to a large, heavy pot over medium-high heat. Clip on a thermometer, if available, and heat to 350°F. Keep the oil temperature as consistent here as possible!
Use tongs to lift the chicken from the marinade and let the excess drip off. Press into the breading on both sides, coating well. Shake off the extra. Carefully lower into the hot oil and fry for 2 minutes without touching. Flip and fry for 2 more minutes. Remove to a wire cooling rack. Immediately top with cheese if using. Repeat with remaining chicken, frying no more than 2 pieces at a time.
Lightly butter the buns and toast in a dry skillet until golden. Spread Chick-Fil-A sauce generously on both sides of the bun. Add lettuce, tomato, and fried chicken. Top with pickles if desired. Place the top bun on and serve right away.
Video
Notes
Storage: Store chicken in an airtight container in the fridge for up to 3 days. Reheat in the oven or air fryer—it helps, but it won’t be as crispy or as fresh. Keep sauce in a separate container for up to 3 days. Assemble sandwiches just before serving.