In a medium bowl, whisk together soy sauce, sake, grated ginger, minced garlic, and sesame oil to create a marinade. Add chicken thighs, ensuring they are well-coated. Cover and refrigerate for at least 30 minutes, or up to 4 hours.

In a shallow dish, combine potato starch and all-purpose flour, stirring well. Set aside for the next step.

In a deep pan or wok, heat vegetable oil to 170°C (340°F). Use a kitchen thermometer to maintain this temperature.

Remove marinated chicken from the fridge and dredge in the flour and potato starch mixture, ensuring a thorough coating.

Lower the coated chicken pieces into the hot oil in batches, frying each for 4-5 minutes until golden brown and crispy.

After frying, let the chicken rest on paper towels to absorb any remaining oil. Serve with lemon wedges and Japanese mayonnaise if desired.