Heat 4 cups vegetable oil in a deep-fryer or large saucepan to 375 degrees F (190 degrees C).
Beat the egg in a mixing bowl. Add the chicken cubes; sprinkle with salt, 1 teaspoon sugar, and white pepper; mix well.
Mix in 1 cup of cornstarch a little bit at a time until the chicken cubes are well coated.
In batches, carefully drop the chicken cubes into the hot oil one by one, cooking until they turns golden brown and begin to float, about 3 minutes. Remove the chicken 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. Cook until the chicken turns deep golden brown, about 2 minutes more. Drain on a paper towel-lined plate.
Heat 2 tablespoons vegetable oil in a wok or large skillet over high heat.
Stir in the green onion, garlic, whole chillies, and orange zest. Cook and stir a minute or two until the garlic has turned golden and the chilies brighten.
Add 1/2 cup sugar, the ginger, chicken broth, vinegar, soy sauce, sesame oil, and peanut oil; bring to a boil and cook for 3 minutes.
Dissolve 2 teaspoons of cornstarch into the water, and 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 the chicken into the boiling sauce. Reduce heat to low and cook for a few minutes until the chicken absorbs some of the sauce.