Prepare the chicken: Place each chicken breast between two sheets of cling film. Using a rolling pin or meat mallet, gently pound to an even thickness of about 1.5cm. You should feel the meat yielding and flattening under the pressure. This ensures even cooking and a tender bite.

Create the brine: In a large bowl, whisk together the pickle juice, milk, and egg until completely combined. The mixture will look pale and slightly frothy. Submerge the pounded chicken breasts in the brine, cover, and refrigerate for at least 30 minutes, or up to 2 hours. The chicken will absorb the tangy liquid, becoming noticeably more tender.

Build the seasoned coating: In a shallow dish, combine the plain flour, cornflour, icing sugar, smoked paprika, garlic powder, onion powder, cayenne pepper (if using), salt, and black pepper. Whisk thoroughly until the spices are evenly distributed. The mixture will smell aromatic and slightly sweet from the paprika and sugar.

Dredge the chicken: Remove one chicken breast from the brine, letting any excess drip off. Place it into the flour mixture, pressing down firmly. Turn it over and press again, ensuring every surface is completely coated. You should see a thick, shaggy layer of coating forming. Shake off any loose excess and place the coated breast on a wire rack. Repeat with the remaining chicken. Let them rest for 5 minutes — this helps the coating adhere.

Heat the oil: Pour the oil into a large, heavy-bottomed pot or deep skillet, filling to a depth of about 5cm. Heat over medium-high heat until the oil reaches 175°C (350°F) on a deep-fry thermometer. If you don't have a thermometer, drop a small pinch of flour into the oil; it should sizzle immediately and vigorously.

Fry the chicken: Gently slide one or two chicken breasts into the hot oil, being careful not to crowd the pan. You should hear a loud, satisfying sizzle as they hit the oil. Fry for 3-4 minutes per side, until the crust is a deep golden brown and the internal temperature of the chicken reaches 74°C (165°F). The crust will feel hard and crisp to the touch. Transfer to a wire rack set over a baking sheet and sprinkle immediately with a tiny pinch of salt.

Toast the buns and assemble: While the chicken rests for a minute, butter the cut sides of the brioche buns. Place them butter-side down in a separate dry skillet over medium heat for about 1 minute, until they are lightly toasted and smell nutty. Spread a little mayonnaise on the top half of each bun, then place the fried chicken on the bottom half. Top with two pickle slices and close the sandwich. Serve immediately while the chicken is still audibly crunchy.