In a large bowl, whisk together buttermilk, dill pickle juice, egg, salt, and pepper until well combined. Add the chicken pieces and refrigerate for at least 2 hours, ideally overnight.

In a shallow dish, combine all-purpose flour, cornstarch, dried dill, baking powder, salt, and pepper, mixing thoroughly.

Remove chicken from marinade, allowing excess to drip off. Dredge chicken in the flour mixture, ensuring thorough coating.

Heat vegetable oil in a heavy pot to 350°F (175°C). Use a thermometer for accurate readings.

Fry chicken pieces for about 6-8 minutes per side until golden brown and internal temperature reaches 165°F (74°C).

Remove chicken and drain on a wire rack or paper towels to keep it crispy.

In a bowl, mix mayonnaise, minced garlic, lemon juice, Dijon mustard, salt, and pepper until smooth. Chill for at least 30 minutes.

Brush cut sides of brioche buns with melted butter and toast in a skillet until golden brown.

Spread dill aioli on both sides of buns, then layer with lettuce, chicken, cheese, and pickle chips before assembling the sandwich.