01 - Whisk together buttermilk, dill, garlic powder, onion powder, salt, and black pepper in a large bowl. Add chicken breasts, turning to coat thoroughly. Cover and refrigerate for at least 30 minutes, up to 4 hours for optimal flavor absorption.
02 - In a shallow dish, combine flour, cornstarch, smoked paprika, salt, and black pepper. Whisk 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 rest for 5 minutes to help breading adhere.
04 - Heat oil in 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, reaching internal temperature of 165°F. Transfer to paper towels to drain excess oil.
05 - Combine mayonnaise, dill, lemon juice, Dijon mustard, garlic powder, salt, and pepper in a small bowl. Mix until smooth and thoroughly incorporated.
06 - Lightly toast brioche buns. Spread dill mayo on both halves. Layer shredded lettuce and pickle slices on bottom bun, place crispy chicken on top, add tomato slices if desired, and cover with top bun.
07 - Serve immediately while chicken remains hot and crispy for optimal texture.