Mix the chicken with the egg, soy sauce, oil, garlic, and ginger. Add the cornstarch, flour, baking powder, white pepper, and salt. Coat the chicken evenly. Add a little more flour and cornstarch if the batter feels too thin.
Heat frying oil in a wok or skillet. First fry: Cook the chicken in two batches for about 4 minutes, until pale golden. Remove and set aside. Second fry: Turn up the heat. Return all the chicken to the oil for about 2 minutes, until deep golden and very crisp.
Pour out most of the frying oil. Add all the sauce ingredients except the slurry and sesame oil. Bring to a boil. Stir in the slurry to thicken the sauce into a glaze.
Add the chicken to the sauce and toss until coated. Finish with the sesame oil. Serve with rice and top with sesame seeds or green onions if you like.