In a large bowl, combine the egg, soy sauce, mochiko, cornstarch, sugar, salt, chopped green onions, garlic, and ginger.
Use the flat side of a meat tenderizer to gently flatten the chicken. Slice into about 2 inch pieces. Add the pieces to the marinade and toss to thoroughly coat. Cover and refrigerate for 4 hours to overnight. Remove from the refrigerator 15-30 minutes before frying.
In a large wok, bring 1 inch of vegetable oil to high heat. Once heated and shimmering, reduce heat to medium.
Gently place 7-9 pieces of chicken into the oil. Be careful not to overcrowd. Fry for 2 1/2-3 minutes per side, until golden and cooked through. Use a slotted spoon to remove the chicken and drain on paper towel-lined plate. Remove any pieces of batter between batches of chicken.
Top with green onions and (optionally) sesame seeds and serve on its own or paired with rice, shredded cabbage, and/or macaroni salad.