In a bowl, mix cornstarch and flour together. Add chicken pieces and toss to coat evenly.
Heat vegetable oil in a pan over medium heat. Fry the chicken pieces until golden and crispy. Remove and drain on paper towels.
In another pan, combine soy sauce, orange juice, orange zest, sugar, rice vinegar, minced ginger, and minced garlic. Stir until the sugar dissolves.
Simmer the sauce over medium heat until it slightly thickens.
Add the fried chicken pieces to the sauce and toss until well-coated.
Serve the orange chicken hot, garnished with chopped green onions or sesame seeds if desired.