Mix salt, black pepper, chilli powder, garlic powder, and paprika powder. Divide the mix into two halves—one for the marinade and one for the coating.

Whisk together eggs, vinegar, and ½ of the spice mix. Add the chicken thighs and coat well. Cover and marinate for at least 1 hour (or refrigerate for 4 hours).

In a separate bowl, mix together plain flour, optional cornflour, and the remaining ½ of the spice mix. Fill another bowl with cold water for dipping.

Remove a piece of chicken from the marinade, letting excess drip off. Dredge the chicken in the flour mixture, pressing down for an even coating. Dip into cold water, fully submerging it. Dredge it again in the flour mixture, pressing down firmly. Shake off excess flour and let it rest for 5 minutes.

Heat oil to 170°C in a deep pan. Fry in batches for 6-8 minutes, flipping occasionally, until golden brown and crispy. Drain on a wire rack (not paper towels).

Toast the burger buns. Spread mayonnaise on both sides. Add shredded lettuce and place a crispy chicken piece on top. Add a cheese slice if desired. Cover with the top bun and serve immediately.