01 - In a large bowl, combine buttermilk, dill, garlic powder, onion powder, salt, and black pepper. Add chicken breasts, turning to coat thoroughly. Cover and marinate in the refrigerator for at least 30 minutes, up to 4 hours for optimal flavor penetration.
02 - In a shallow dish, whisk together flour, cornstarch, paprika, salt, and black pepper until evenly distributed.
03 - Remove chicken from marinade, allowing excess liquid to drip off. Press each piece firmly into the flour mixture to ensure complete coating. Arrange on a tray and let rest for 5 minutes to set the breading.
04 - Heat oil in a deep fryer or large skillet to 350°F. Fry chicken breasts in batches for 4–5 minutes per side until golden brown and cooked through (internal temperature reaches 165°F). Transfer to paper towels to drain excess oil.
05 - In a small bowl, whisk together mayonnaise, dill, lemon juice, Dijon mustard, garlic powder, salt, and pepper until smooth and well combined.
06 - Lightly toast brioche buns. Spread dill mayo generously on both halves. Layer shredded lettuce and pickles on the bottom bun, place crispy chicken on top, add tomato slices if desired, and cover with the top bun.
07 - Serve immediately while chicken is hot and coating maintains maximum crunch.