Step 1: Prepare the chicken. In a shallow dish, combine the all-purpose flour, 1/4 cup cornstarch, salt, and black pepper. Dredge the chicken pieces, ensuring they are fully coated. Set aside.
Step 2: Heat oil. In a large wok or deep skillet, heat the vegetable oil over medium-high heat to 350-375°F (175-190°C).
Step 3: Fry the chicken. Fry the chicken in batches, about 3-4 minutes per batch, until golden brown and cooked through. Avoid overcrowding the pan. Remove chicken with a slotted spoon and place on a wire rack over paper towels to drain excess oil. For extra crispiness, you can double-fry by letting chicken rest for a few minutes, then frying again for 1-2 minutes until deeply golden.
Step 4: Prepare the sauce. In a saucepan, combine the fresh orange juice, orange zest, rice vinegar, soy sauce, granulated sugar, brown sugar, grated ginger, minced garlic, and red pepper flakes. Bring to a simmer over medium heat, stirring until sugars dissolve.
Step 5: Thicken the sauce. Whisk in the cornstarch slurry and continue to simmer, stirring constantly, until the sauce thickens to your desired consistency, about 1-2 minutes.
Step 6: Combine and serve. Add the fried chicken to the saucepan with the sauce, tossing gently to coat evenly. Serve immediately over steamed white rice, garnished with green onions and sesame seeds if desired.