Cut chicken into 1-inch cubes.
In a bowl, mix flour, cornstarch, salt, and pepper. Add egg, oil, and water, whisk until smooth.
Coat chicken thoroughly in the batter. Cover and chill for 30 minutes.
Heat oil to 350°F (177°C).
Fry chicken in batches for 2–3 minutes until golden and crispy. Remove and keep warm.
In a saucepan, combine orange juice, zest, soy sauce, vinegar, sugar, and garlic. Bring to a low boil.
Add cornstarch slurry and stir until thickened.
Toss fried chicken in the sauce until fully coated. This Chinese Orange Chicken becomes glossy and rich at this stage.
Garnish and serve immediately.