In a large mixing bowl, combine flour, cornstarch, baking powder, salt, and black pepper. Mix well.
Add the chicken pieces to the dry mixture and toss until well coated.
Heat vegetable oil in a frying pan or wok over medium-high heat. Once hot, carefully add the chicken in batches, frying until golden brown and cooked through, about 5-6 minutes per batch. Use a cooking thermometer to ensure the internal temperature reaches 165°F (74°C). Remove chicken and drain on paper towels.
In a saucepan, combine orange juice, orange zest, sugar, soy sauce, rice vinegar, minced garlic, and minced ginger. Bring to a simmer over medium heat.
Stir in the cornstarch-water mixture and cook for an additional 1-2 minutes, or until the sauce thickens.
Toss the fried chicken pieces in the sauce until well coated.
Serve the Orange Chicken immediately, garnished with chopped green onions and sesame seeds if desired. Enjoy with steamed rice!