Cut each chicken thigh fillet into two-inch cubes.
In a mixing bowl, whisk together the egg white, starch, salt and pepper. Add the chicken cubes and mix. Cover the bowl and keep in the refrigerator while you make the orange sauce.
Pour the sugar, orange juice. rice wine vinegar and soy sauce into a small non-reactive sauce pan. Stir in the orange zest and grated ginger. Boil over medium heat for 10 to 15 minutes or until reduced by half. The sauce will thicken as it cools.
While the orange sauce reduces, heat the cooking oil in a wok or frying pan until fine wisps of smoke float on the surface.
Drop the chicken cubes, one at a time. Fry over medium heat until golden brown, about five minutes. Scoop out and drain on a stack of paper towels.
Pour off the oil from the frying pan or wok.
Pour the orange sauce into the wok. Add the chicken cubes and toss to coat every piece.
Transfer the orange chicken to a serving plate. Spoon whatever sauce is left over and around it. Sprinkle with toasted sesame seeds and sliced scallions. Serve at once.