In a large bowl, whisk together the cornstarch and egg whites with a fork until almost frothy, about 1 minute. Add the chicken to the mixture and allow to sit for 5 to 10 minutes.
For the sauce:
Meanwhile, put the orange juice, soy sauce, sugar, vinegar, sesame oil, salt, crushed red pepper, garlic and ginger (and orange zest, if using) in a small nonstick skillet and whisk together. Heat until bubbling and starting to thicken, about 3-4 minutes.
Whisk together the cornstarch and 1/4 cup water in a small bowl and add 1 to 2 tablespoons of the cornstarch slurry to the sauce. Mix in and thicken for 1 minute. (If sauce gets overly thick, just add in another 1/4 cup water and whisk in.)
Heat about 2 inches of vegetable oil in a heavy-bottomed pot until a deep-fry thermometer inserted in the oil registers 350 degrees F. In batches, carefully drop a few pieces of chicken into the oil (drop them in one by one to keep them from sticking together) and move it around, 2-3 minutes or until light golden.
Let the pieces drain on a plate lined with paper towels for 2 to 3 minutes. Then drop them back into the oil for 1 minute to really solidify the coating.
Toss the chicken in the sauce and serve immediately with orange zest and sliced green onions on the top.