01 - Pound chicken breasts between parchment sheets to uniform 1/2-inch thickness for even cooking.
02 - Whisk buttermilk with salt, pepper, garlic powder, and paprika. Submerge chicken and refrigerate 30 minutes to 4 hours.
03 - Combine flour, cornstarch, salt, paprika, and cayenne in a shallow dish for the coating mixture.
04 - Pour oil into deep skillet or Dutch oven to 2-inch depth. Heat to 350°F.
05 - Remove chicken from marinade, shake off excess liquid. Press firmly into flour mixture, ensuring thick, even coating on all sides.
06 - Fry chicken 4-5 minutes per side until deep golden brown and internal temperature reaches 165°F. Drain on wire rack.
07 - Stir together mayonnaise and Dijon mustard until smooth and fully incorporated.
08 - Spread sauce on both bun halves. Layer lettuce, fried chicken, pickles, and tomato. Crown with top bun and serve immediately.