To marinate the chicken, in a bowl, add the chicken pieces and salt and mix until absorbed. Add the white pepper and rice wine and mix well. In a small bowl, combine the cornstarch and water to form a slurry, then add it along with the egg white to the chicken. Mix thoroughly and let marinate while preparing the remaining ingredients.
To make the sauce, in a bowl, whisk together the orange juice, soy sauce, ketchup, rice vinegar, sugar, and orange zest until well combined.
To make the batter, in a large bowl, whisk together the cornstarch, baking soda, baking powder, salt, egg, cooking oil, and water until smooth. Add the marinated chicken and coat evenly.
To fry the chicken, in a pot over high heat, add the cooking oil and heat to 350°F (177°C). Fry the chicken in batches for about 5 minutes until golden brown. Remove and drain. Increase the oil temperature to 375°F (191°C), return the chicken, and fry for 1 minute more until extra crispy. Remove and drain well.
To finish the dish, in a pan over medium heat, bring the prepared sauce to a simmer. Stir the cornstarch slurry, then pour it into the sauce and cook until thickened and glossy. Add the fried chicken and toss until evenly coated. Garnish with scallions and orange slices if using, and serve immediately.