In a large bowl, combine soy sauce, sugar, water, sesame oil, garlic powder, onion powder, black pepper, salt, and cayenne pepper. Mix well until the sugar is dissolved.
Add the chicken pieces to the marinade, ensuring they are well coated. Cover and refrigerate for at least 1 hour, or overnight for best flavor.
After marinating, remove the chicken from the refrigerator. In a separate bowl, place the mochiko.
Heat about 1 inch of vegetable oil in a deep skillet or frying pan over medium-high heat.
Take each piece of marinated chicken, dredge it in the mochiko, and shake off any excess.
Once the oil is hot (about 350°F), carefully add the coated chicken pieces in batches, making sure not to overcrowd the pan. Fry for about 5-7 minutes or until golden brown and cooked through, turning occasionally for even cooking.
Remove the chicken with a slotted spoon and place it on a paper towel-lined plate to drain excess oil.
Serve hot, garnished with green onions or sesame seeds if desired.