To make orange sauce:
In a medium pot, add orange juice, sugar, vinegar, soy sauce, ginger, garlic, and red chili flakes. Heat for 3 minutes.
In a small bowl, whisk 1 Tablespoon of cornstarch with 2 Tablespoons of water to form a paste. Add to orange sauce and whisk together. Continue to cook for 5 minutes, until the mixture begins to thicken. Once the sauce is thickened, remove from heat and add orange zest.
To make chicken:
Place flour and cornstarch in a shallow dish or pie plate. Add a generous pinch of salt. Stir.
Whisk eggs in shallow dish.
Dip chicken pieces in egg mixture and then flour mixture. Place on plate.
Heat 2 -3 inches of oil in a heavy-bottomed pot over medium-high heat. Using a thermometer, watch for it to reach 350 degrees.
Working in batches, cook several chicken pieces at a time. Cook for 2 - 3 minutes, turning often until golden brown. Place chicken on a paper-towel-lined plate. Repeat.
Toss chicken with orange sauce. You may reserve some of the sauce to place on rice. Serve it with a sprinkling of green onion and orange zest, if so desired.