01 - Place chicken breasts between two sheets of parchment paper 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 refrigerator for at least 30 minutes up to 4 hours.
03 - In a shallow dish, combine flour, cornstarch, salt, paprika, and cayenne pepper until well blended.
04 - Heat vegetable oil in a deep skillet or Dutch oven to 350°F (about 2 inches deep).
05 - Remove chicken from marinade, shaking off excess liquid. Dredge each piece in the flour mixture, pressing firmly to ensure thick, even coating.
06 - Fry chicken for 4-5 minutes per side until golden brown and cooked through to internal temperature of 165°F. Drain on wire rack or paper towels.
07 - Mix mayonnaise and Dijon mustard in a small bowl until smooth and combined.
08 - Spread sauce on both halves of each toasted bun. Layer lettuce, fried chicken, pickles, and tomato. Place bun lid on top and serve immediately.