01 - Combine buttermilk, dill, garlic powder, onion powder, salt, and black pepper in a large bowl. Add chicken breasts, turning to coat evenly. 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 thoroughly combined.
03 - Remove chicken from marinade, allowing excess liquid to drip off. Press each piece firmly into the flour mixture to ensure complete coating. Place breaded chicken on a tray and let rest for 5 minutes to set the coating.
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 an internal temperature of 165°F. Transfer to paper towels to drain excess oil.
05 - Mix mayonnaise, fresh 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 on both halves. Layer shredded lettuce and pickles on bottom bun, place crispy chicken on top, add tomato slices if desired, and cover with top bun.
07 - Serve sandwiches immediately while chicken remains hot and crispy for optimal texture.