Prepare the chicken: Place each chicken thigh between two sheets of baking paper and gently flatten with a rolling pin until about 1cm thick. You want them even so they cook at the same speed. The chicken should feel slightly plump but uniform under your fingers.

Wet the chicken: Pour the buttermilk into a shallow bowl. Add the chicken pieces, turning to coat each one thoroughly. Let them sit for 5 minutes while you prepare the dry mix. The buttermilk should look creamy and cling to the chicken like a thick coating.

Make the seasoned flour: In a separate wide bowl, whisk together the plain flour, cornflour, salt, smoked paprika, garlic powder, and white pepper. The mix should smell fragrant and look a pale golden colour from the paprika.

Dredge the chicken: One piece at a time, lift a thigh from the buttermilk, letting any excess drip off. Drop it into the seasoned flour and press down firmly with your fingers. Turn and press again until the chicken is completely coated. You should hear a light rustling sound as the flour clings to the wet surface. Place the breaded chicken on a plate.

Heat the oil: Pour enough vegetable oil into a large frying pan to reach a depth of about 1cm. Heat over a medium-high flame until the oil shimmers and a pinch of flour sizzles immediately on contact. The oil should smell clean and hot, not smoky. This takes about 2 minutes.

Fry the chicken: Gently lay the breaded thighs into the hot oil, leaving space between each piece. You will likely need to do this in two batches. Fry for 4 minutes on the first side. The underside should turn a deep golden brown and the crust will look crackled. Listen for a steady, gentle sizzle — if it's too loud, lower the heat slightly. Flip with tongs and fry for another 3–4 minutes. The chicken should feel firm when pressed and the crust should look dark amber and crunchy.

Rest the chicken: Transfer the fried chicken to a wire rack set over a baking tray. Do not put it on paper towels — the steam will soften the crust. Let it rest for 3 minutes. During this time, the chicken will continue cooking inside and the crust will set to a shatteringly crisp finish.

Assemble the sandwiches: Spread 1 tablespoon of mayonnaise on the cut side of each toasted bun. Place a lettuce leaf on the bottom half, then add a piece of fried chicken. Top with pickled cucumber slices if using, then close the sandwich with the top bun. Serve immediately while the chicken is still hot and the bun is soft.