Heat vegetable oil in a deep fryer or large saucepan to 375 degrees F (190 degrees C).

Whisk egg in large mixing bowl. Add chicken, salt, sugar, and pepper; mix well. Mix in cornstarch, a little bit at a time, until chicken is well coated.

Working in batches, carefully drop one piece of chicken at a time into the hot oil. Fry until chicken turns golden brown and begins to float, about 3 minutes. Remove chicken to a plate and allow to cool as you fry the next batch.

Once all of the chicken has been fried, refry the chicken, starting with the batch that was cooked first. Fry each batch until chicken turns deep golden brown, about 2 minutes more. Drain on a paper towel-lined plate.

To make the sauce: Heat vegetable oil in a wok or large skillet over high heat. Stir in green onion, garlic, whole chiles, and orange zest. Cook and stir until garlic turns golden and chiles brighten, 1 to 2 minutes.

Add sugar, soy sauce, chicken broth, peanut oil, rice vinegar, sesame oil, and ginger; bring to a boil and cook for 3 minutes.

Whisk 2 teaspoons of cornstarch in water until dissolved. Stir into the boiling sauce. Return to a boil and cook until the sauce thickens and is no longer cloudy from the cornstarch, about 1 minute.

Stir chicken into the boiling sauce. Reduce heat to low and cook until chicken is heated through and has absorbed some of the sauce, about 3 minutes.