01 - Combine 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 infusion.
02 - Whisk together flour, cornstarch, paprika, salt, and black pepper in a shallow dish until evenly distributed.
03 - Remove chicken from marinade, allowing excess liquid to drip off. Press each piece firmly into the flour mixture, ensuring complete coating. Arrange breaded chicken on a tray and rest for 5 minutes to set the breading.
04 - Heat oil to 350°F in a deep fryer or large skillet. Cook chicken breasts in batches for 4–5 minutes per side until golden brown and internal temperature reaches 165°F. Transfer to paper towels to drain excess oil.
05 - Whisk mayonnaise, dill, lemon juice, Dijon mustard, garlic powder, salt, and pepper in a small bowl until smooth and well combined.
06 - Lightly toast brioche buns. Spread dill mayo generously on both halves. Layer lettuce and pickles on bottom bun, place crispy chicken on top, add tomato slices if desired, and crown with top bun.
07 - Serve immediately while chicken remains hot and crispy for optimal texture.