For the orange chicken sauce
1/4 cup white vinegar
1/4 cup sugar
2 tablespoons water
2 tablespoons soy sauce
1 teaspoon cornstarch
1/2 teaspoon orange extract
1/2 teaspoon salt
orange zest if desired
For the chicken
1 egg
1 teaspoon salt
1/2 teaspoon ground white pepper
1 tablespoon vegetable oil plus additional for frying
1 cup cornstarch
1/2 cup all-purpose flour
1 pound boneless skinless chicken thighs cut into 1-inch pieces
vegetable oil for frying
2 tablespoons sesame oil
4 to 6 dried tianjin peppers
2 teaspoons chili flakes
1 teaspoon minced garlic
1/2 teaspoon minced ginger
To make the sauce, whisk the vinegar, sugar, water, soy sauce, cornstarch, orange extract, salt, and orange zest (if using) in a small bowl. Set aside.
To make the chicken, whisk the egg, salt, white pepper and vegetable oil in a large bowl. In a second bowl, stir together the cornstarch and flour.
Dip the chicken pieces into the egg mixture. Shake off any excess and transfer the chicken to the cornstarch mixture. Press down lightly so the cornstarch adheres on all sides.
In a deep fryer or Dutch oven, heat four inches of vegetable oil to 350°F.
Working in batches, drop the chicken pieces into the oil one at a time. Deep-fry until the breading turns golden brown, 3 to 4 minutes or until the chicken reaches an internal temperature of 170°. Remove the chicken to a rack to drain.
When nearly all the chicken is finished frying, heat the sesame oil in a large skillet over medium heat. Add the dried chilis and cook until lightly toasted, about 1 minute. Add the chili flakes and cook until fragrant, about 1 minute. Add the garlic and ginger, and cook until fragrant, about 1 minute.
Add the reserved orange chicken sauce to the skillet and bring the mixture to a simmer. Cook, stirring, until the sugar is dissolved and the sauce has thickened, 1 to 2 minutes.
Add the chicken to the skillet and toss to coat. Serve immediately.