Cut chicken thighs into 1–2 inch cubes and set aside.
In a medium bowl combine salt, white pepper, 1 cup cornstarch, and 3 cups flour; whisk to blend.
Whisk in the egg, 1½ cups water, and 2 tablespoons vegetable oil until a thick batter forms.
Toss the chicken cubes in the batter to coat, then refrigerate for 30 minutes.
Heat 3 cups vegetable oil in a deep fryer or large pot to 350°F (175°C).
Fry the battered chicken in batches (do not overcrowd) until golden and cooked through, about 4–6 minutes per batch; drain on paper towels.
For the sauce, heat 1 tablespoon vegetable oil in a saucepan over medium-high heat. Add red pepper flakes, minced ginger, and minced garlic and cook about 30 seconds until fragrant.
Add brown sugar and orange juice, stir to combine, then mix in vinegar and optional soy sauce.
Whisk 2 tablespoons cornstarch with 2 tablespoons water (use the optional water if desired) to make a slurry, then stir into the sauce and cook until thickened.
Add the fried chicken to the sauce and toss to coat evenly, then finish with 2 teaspoons sesame oil and serve.
