Prepare and Brine the Chicken: Place each chicken breast between two sheets of baking parchment and use a rolling pin or meat mallet to gently pound them to a uniform thickness of about 2cm. This ensures they cook evenly. Place the chicken in a bowl, pour over the buttermilk and 1 tsp of salt. Stir to combine, ensuring the chicken is fully submerged. Cover and refrigerate for at least 1 hour, or up to 4 hours.

Mix the Coating and Sauce: While the chicken brines, prepare your other components. In a wide, shallow dish, whisk together the plain flour, cornflour, smoked paprika, garlic powder, onion powder, cayenne pepper, black pepper, and 2 tsp salt. In a separate small bowl, mix all the ingredients for the smoky mayonnaise. Cover and set aside.

Heat the Oil: Pour the oil into a large, heavy-bottomed saucepan or Dutch oven until it is about 3-4 cm deep. Heat over a medium-high heat until it reaches 175°C (350°F). A food thermometer is highly recommended here for accuracy. If you don't have one, a cube of bread should turn golden brown in about 30 seconds.

Dredge the Chicken: Set up your dredging station. Take one piece of chicken out of the buttermilk, allowing excess to drip off, but don't wipe it dry. Press it firmly into the flour mixture, ensuring it's coated all over. Dip it back into the buttermilk for a second, then press it back into the flour mixture for a final, thick coating. I find that using one hand for the wet buttermilk and the other for the dry flour mixture helps prevent your fingers from getting caked in batter. Place the coated chicken on a wire rack. Repeat with the remaining chicken.

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

Rest the Chicken: Once cooked, remove the chicken from the oil using tongs and place it on a clean wire rack to drain. Sprinkle immediately with a tiny pinch of extra salt. Let it rest for a few minutes while you fry the remaining two pieces.

Toast the Buns: While the second batch of chicken is frying, split your brioche buns. You can either toast them in a dry pan until lightly golden or briefly place them under a hot grill.

Assemble Your Sandwich: Spread a generous layer of the smoky mayonnaise on the top and bottom halves of each toasted bun. Place a piece of crispy fried chicken on the bottom bun, top with a few leaves of crisp lettuce and several slices of pickle. Place the top bun on and serve immediately.