Brine the Chicken: In a medium bowl, whisk together the buttermilk, pickle juice, and 1 teaspoon of salt. Place the chicken breasts between two sheets of cling film and gently pound them to an even thickness of about 2cm. This ensures they cook evenly. Add the chicken to the brine, ensuring it's fully submerged. Cover and refrigerate for at least 1 hour, or up to 4 hours.

Prepare the Coating: While the chicken brines, prepare your dredging station. In a wide, shallow dish, whisk together the plain flour, cornflour, icing sugar, paprika, garlic powder, onion powder, black pepper, cayenne pepper, and 1 teaspoon of salt. Mix until everything is thoroughly combined.

Coat the Chicken: Remove one piece of chicken from the brine, allowing any excess to drip off. Dredge it thoroughly in the seasoned flour mixture, pressing down gently to ensure the flour adheres all over. Dip it back into the buttermilk brine for a second, then dredge it once more in the flour. This double-dredge is key for an extra-crispy coating. Set the coated chicken on a wire rack and repeat with the remaining fillets.

Rest the Chicken: Let the coated chicken rest on the wire rack for 15-20 minutes. I find this step is essential for helping the coating set and stick to the chicken during frying.

Heat the Oil: Pour the oil into a large, heavy-bottomed pan or Dutch oven to a depth of about 4-5cm. Heat over a medium-high heat until it reaches 175°C (350°F). Use a cooking thermometer for accuracy.

Fry the Chicken: Carefully place two chicken fillets into the hot oil, ensuring not to overcrowd the pan. Fry for 4-6 minutes on each side, until the coating is a deep golden brown and the chicken is cooked through. The internal temperature should reach 74°C (165°F).

Drain and Rest: Remove the cooked chicken from the oil and place it back on the wire rack to drain any excess oil. Let it rest for a few minutes while you fry the remaining chicken.

Assemble the Sandwiches: While the chicken rests, lightly butter the cut sides of your brioche buns and toast them in a dry frying pan until golden. Spread a generous layer of mayonnaise on the top and bottom bun, add the crispy chicken fillet, top with a few slices of dill pickle, and place the top bun on. Serve immediately.