1. In a large bowl, combine the chicken pieces, beaten egg, cornstarch, flour, salt, and pepper. Mix well until the chicken is evenly coated. Let the chicken marinate for at least 15 minutes at room temperature, or up to an hour in the refrigerator.

2. In a medium saucepan, whisk together the orange juice, rice vinegar, soy sauce, granulated sugar, brown sugar, orange zest, sesame oil, ginger, garlic, and red pepper flakes (if using).

3. In a small bowl, combine the cornstarch and water to create a slurry.

4. Bring the orange sauce mixture to a simmer over medium heat. Slowly whisk in the cornstarch slurry. Continue to simmer, stirring constantly, until the sauce has thickened to your desired consistency, about 2–3 minutes. Taste and adjust seasonings if needed. Remove the sauce from the heat and set aside.

5. Pour enough vegetable oil into a large, deep pot or Dutch oven to reach a depth of about 3–4 inches. Heat the oil over medium-high heat to 350°F (175°C).

6. Carefully add the chicken pieces to the hot oil in batches, being careful not to overcrowd the pot. Fry the chicken for about 5–7 minutes per batch, or until golden brown and cooked through. The internal temperature of the chicken should reach 165°F (74°C).

7. Remove the fried chicken with a slotted spoon and place it on a wire rack lined with paper towels to drain excess oil.

8. In a large bowl, gently toss the fried chicken with the prepared orange sauce until evenly coated.

9. Serve immediately over rice or noodles. Garnish with sesame seeds, chopped green onions, and orange slices, if desired.