There are many versions of this classic Japanese dish. It's really delicious and served at many Japanese restaurants, usually served over white rice. If you can find it at an Asian market, it's better to use Chinese soup powder in place of the chicken bouillon powder as it has a distinct Asian flair to it. But it's not always available in all Asian markets, in which case it's fine to use chicken bouillon instead.
Mix the soy sauce, sake, ginger, garlic, bouillon, and green onion in a small bowl. Add chicken pieces, and marinate in refrigerator for about 30 minutes.
Remove chicken from the marinade (discarding marinade). Add chicken and katakuriko/cornstarch to a bowl, and mix well (until all chicken pieces are evenly coated).
Heat oil to 375°F (190°C), and fry chicken until cooked through, about 5 minutes. You might have to do this in batches, as you don't want to fry too many pieces at once as it will not get crispy enough.
Transfer chicken to paper towel-lined plate to absorb oil. Repeat until all chicken is fried.