In a bowl, combine together minced garlic, ginger, green onions, soy sauce, mirin, potato starch, five spices powder, white pepper and cayenne pepper. Add the chicken pieces and mix to coat. Cover with plastic wrap and refrigerate for at least half an hour or overnight.
Beat the egg in a medium sized bowl with the ½ cup of water. Spread the potato starch in a shallow dish.
Heat the frying oil in a sauté pan or wok until it reaches 350˚F. Using chopsticks or tongs, dip the chicken first in the watery egg mixture then in the starch and fry the chicken pieces without crowding them too much for 3-4 minutes or until golden brown. Drain the browned chicken on a wire rack or a plate lined with paper towels. Immediately toss with generous amounts of salt and white pepper to taste.