Set the chicken aside.Place the chicken thighs between 2 sheets of baking paper. Use a rolling pin (or heavy object) to press down and flatten the chicken pieces.
Whisk the milk, vinegar, egg and 2 tbs of the spice mix in a large bowl. Add the chicken pieces and place in the fridge to marinate for a minimum of 30 minutes (or up to 12 hours).
Make the crispy coating by mixing the self raising flour, cornflour and 1 tbs of the reserved spice mix together in a bowl. Drain the chicken thighs, and reserve the marinade. Dip the chicken pieces into the flour mixture then into the marinade.
Place a large, deep frying pan with vegetable oil over high heat. Cook the chicken thighs (in batches) for 4-5 minutes on either side or until golden and cooked through.