Remove bones from chicken thigh by gently running kitchen knife down either side to loosen the flesh and any tiny tendons. Wash thoroughly and cut thighs in roughly 2 inches slices.
Transfer chicken in a bowl then add salt, pepper, soy sauce, ginger, garlic, rice vinegar and mirin. Mix well and marinate for 30 minutes.
When ready to cook, heat oil in a deep frying pan over medium-high heat.
While waiting for the oil to heat up, add egg and flour into the bowl with the chicken and marinade. Mix until all chicken pieces are coated with flour.
Place potato starch in a bowl and dredge chicken into the potato starch. Shake off any excess flour and transfer in a baking tray or rack.
When the oil is ready, fry the coated chicken in batches but do not over crowd. When the karaage hits an internal temperature of 160 degrees F (71 C) or golden brown, transfer them in a baking tray lined with paper towels.
Let rest for a couple of minutes then serve and enjoy. Bon appetit!