Combine the soy sauce, sake, garlic, wasabi, ginger, sugar, salt, and pepper in a large sealable bag. Toss in the chicken thighs and mix together until fully coated. Allow to marinate in the refrigerator for at least 1 hour, up to 8 hours.
After the chicken has marinated, combine the potato starch, wasabi powder and ground ginger in a small bowl. Fill a deep heavy pot with 1 ½ inches of peanut-oil and heat to 325°F over medium-high.
Transfer the marinated chicken into a large bowl. Drain as much extra liquid as you can, it is okay if there is some there but we don’t want it soaking. Add the potato starch mixture and mix until all the chicken is covered. It will become pasty and look a bit patchy.
Once the oil has reached the desired temperature, carefully place a few pieces of chicken in the oil, making sure to not overcrowd the pot. Fry for 90 seconds. You will have to do this in several batches. Transfer to a plate covered with paper towels. Repeat with the remaining chicken.
After all the pieces have been cooked, increase the oil’s temperature to 350°F. Refry all the pieces of chicken for 60 seconds. Transfer to a plate covered with paper towels. Repeat with the remaining chicken. The second fry will make all the chicken extra crispy.
