In a large bowl, mix cornstarch, flour, baking powder, water, soy sauce, and garlic powder to make a batter.
Coat chicken pieces in the batter.
Heat oil in a deep skillet and fry chicken pieces until golden and crispy. Remove and set aside.
In a saucepan, combine soy sauce, rice vinegar, sugar, and sesame oil. Heat until sugar dissolves.
Stir in the cornstarch slurry and simmer until the sauce thickens.
Toss fried chicken in the sauce until fully coated.
Garnish with green onions and sesame seeds before serving.