01 - Place chicken breasts between two sheets of parchment and gently pound to even thickness (about 0.6 inch).
02 - In a bowl, mix buttermilk, salt, pepper, garlic powder, and paprika. Add chicken, cover, and marinate in the fridge for at least 30 minutes (up to 4 hours for best flavor).
03 - In a shallow dish, combine flour, cornstarch, salt, paprika, and cayenne pepper.
04 - Heat oil in a deep skillet or Dutch oven to 350°F (about 2 inches deep).
05 - Remove chicken from marinade, shaking off excess. Dredge each piece in the flour mixture, pressing firmly for a thick coating.
06 - Fry chicken for 4–5 minutes per side, until golden brown and cooked through (internal temp 165°F). Drain on a wire rack or paper towels.
07 - Mix mayonnaise and Dijon mustard to create the burger sauce.
08 - Spread the sauce on both halves of each toasted bun. Layer lettuce, fried chicken, pickles, and tomato. Top with the bun lid and serve immediately.