Cut chicken thighs into bite-sized pieces (~1.5–2 inches).
In a bowl, combine soy sauce, sake, mirin, ginger, garlic, and sugar.
Add chicken and mix well. Cover and marinate for 30 minutes to 1 hour (not too long or it can get too salty).
Mix cornstarch and potato starch/all-purpose flour in a shallow bowl.
Remove chicken from the marinade, letting excess drip off.
Coat each piece evenly in the starch mixture, pressing lightly so the starch sticks. For extra crunch, double coat: lightly dip in starch again just before frying.
Heat oil in a deep pan to 180°C.
Fry chicken in batches so you don’t overcrowd the pot. Fry in batches for 4–5 minutes, turning occasionally, until golden and cooked through.
Note: Shake off any excess starch before frying so the coating stays light and doesn’t clump.
For best crispness, double-fry: Increase oil temperature to 190°C and fry the chicken again for 30–60 seconds until deep golden and extra crispy.
Drain on a wire rack or paper towels.
Serve hot with lemon wedges and Japanese mayo if you like!