Prepare the orange sauce.
In a saucepan over medium heat, combine the orange juice, sugar, garlic, red pepper flakes, ginger, soy sauce, and rice vinegar. Stir and bring to a gentle simmer, cooking for about 3 minutes.
Thicken the sauce.
In a small bowl, stir together the water and 1 tablespoon cornstarch until smooth. Slowly stir this mixture into the simmering sauce.
Reduce the heat to low and cook for 3–5 minutes, stirring frequently, until thick and glossy. Remove from the heat and set aside.
Prepare the chicken.
Cut the chicken into evenly sized, bite-size pieces. Pat dry with paper towels.
In one bowl, whisk the eggs until combined. In a separate bowl, mix the flour and 1/2 cup cornstarch.
Coat the chicken.
Dip each piece of chicken into the eggs, letting the excess drip off. Toss in the flour mixture until evenly coated, then shake off any excess.
Fry in batches.
Pour about 2 inches of vegetable oil into a deep, heavy pot, leaving plenty of room above the oil. Heat to 350°F.
Carefully add the chicken in small batches without crowding the pot. Fry for approximately 4–6 minutes, turning as needed, until golden and cooked through. The center of the chicken should reach 165°F.
Transfer to a wire rack set over a baking sheet. Allow the oil to return to 350°F between batches.
Toss and serve.
Gently rewarm the orange sauce if needed. Toss the cooked chicken with the sauce just before serving, coating each piece evenly. Serve immediately over rice.