Marinate the chicken with soy sauce, vinegar, sesame oil, ginger, garlic, egg, and water for 15–20 minutes.
Mix cornstarch, flour, baking powder, salt, and pepper. Coat the chicken evenly.
Fry in batches at 350°F (175°C) for approximately 4–6 minutes per batch, until golden and the chicken reaches 165°F (74°C) internally.
Simmer the sauce ingredients in a separate skillet. Add the cornstarch slurry and stir until glossy and thickened.
Toss the crispy chicken with the sauce, garnish with sesame seeds and green onions, and serve immediately.