Prepare the Coating: In one bowl, whisk together flour, cornstarch, salt, black pepper, garlic powder, and ginger powder. In another bowl, beat the egg and gradually add water to make a thin batter consistency.
Coat the Chicken: Dip each piece of chicken into the egg mixture, then coat in the flour mixture. Set aside on a plate lined with paper towels.
Heat the Oil: Heat 2 inches of vegetable oil in a heavy-bottomed pot or wok to 350°F (175°C). Test the temperature with a small piece of batter—if it sizzles immediately, the oil is ready.
Fry the Chicken: Fry chicken pieces in batches for 4-5 minutes or until golden brown and cooked through. Remove from oil and drain on paper towels.
Make the Orange Sauce: In a saucepan, combine orange juice, soy sauce, sugar, rice vinegar, and sesame oil. Bring to a gentle boil and stir until the sugar dissolves. Add orange zest and simmer for 2-3 minutes.
Thicken the Sauce: In a small bowl, mix cornstarch and water to make a slurry. Slowly pour it into the simmering sauce, whisking constantly. The sauce will thicken in 1-2 minutes.
Combine and Serve: Add fried chicken to the sauce and toss gently to coat. Serve immediately, garnished with sliced green onions and sesame seeds. Serve with steamed rice and vegetables.