Whisk the sauce: combine 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, and orange zest (if using) in a small bowl until smooth; set aside.
Prepare the dredging station: in a large bowl whisk the egg with 1 teaspoon salt, 1/2 teaspoon ground white pepper, and 1 tablespoon vegetable oil. In a second large bowl stir together 1 cup cornstarch and 1/2 cup all-purpose flour.
Coat the chicken: dip 1-inch chicken pieces into the egg mixture, shake off excess, then press into the cornstarch-flour mixture to coat evenly; set aside on a tray while you heat the oil.
Heat 3–4 inches of vegetable oil in a deep fryer or Dutch oven to 350°F (175°C). Working in batches, fry chicken pieces 3 to 4 minutes until golden and a thermometer reads 165°F; transfer to a wire rack to drain.
Toast aromatics: when most of the chicken is done, heat 2 tablespoons sesame oil in a large skillet over medium heat. Add 4–6 dried Tianjin peppers and cook about 1 minute until lightly toasted.
Add 2 teaspoons chili flakes and cook 30–60 seconds until fragrant, then add 1 teaspoon minced garlic and 1/2 teaspoon minced ginger and cook another 30–60 seconds until aromatic.
Cook the sauce: pour the reserved orange sauce into the skillet and bring to a simmer, stirring until sugar dissolves and sauce thickens, about 1–2 minutes.
Toss and serve: add the fried chicken to the skillet and toss to coat thoroughly with the sauce; serve immediately.
