01 - In a large bowl, combine buttermilk, dill, garlic powder, onion powder, salt, and black pepper. Add chicken breasts, turning to coat. Cover and marinate in the fridge for at least 30 minutes (or up to 4 hours for best flavor).
02 - In a shallow dish, whisk together flour, cornstarch, paprika, salt, and black pepper until evenly combined.
03 - Remove chicken from marinade, letting excess drip off. Dredge each piece in the flour mixture, pressing firmly to adhere coating. Place on a tray and let sit 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, or until golden brown and cooked through (internal temperature reaches 165°F). Drain on paper towels.
05 - In a small bowl, mix together mayonnaise, dill, lemon juice, Dijon mustard, garlic powder, salt, and pepper until smooth and well combined.
06 - Toast brioche buns lightly. Spread dill mayo on both halves. Layer lettuce and pickles on the bottom bun, add crispy chicken, and top with optional tomato slices. Cover with top bun.
07 - Serve immediately while hot and crispy for best texture and flavor.