In a large bowl, whisk together flour, cornstarch, salt, and white pepper. Add the egg, water, and 1 1/2 tablespoons of oil, mixing until the batter is smooth and sticky. Toss the chicken pieces in the batter, ensuring they are evenly coated. Refrigerate the coated chicken while heating the oil for frying. This resting time helps the batter adhere better to the chicken during cooking.
Heat 2 inches of oil in a deep saucepan to 375°F (190°C) for frying. In the meantime, prepare the sauce by heating the remaining oil and 2 teaspoons of sesame oil in a large skillet over medium-high heat. Add the crushed red pepper, ginger, and garlic, cooking for about 30 seconds until fragrant. Then, stir in the brown sugar, orange juice, vinegar, and soy sauce, bringing the mixture to a boil while stirring frequently. This step allows the flavors to meld and the sauce to thicken slightly.
In a small bowl, mix 3 tablespoons of cold water with cornstarch until dissolved. Pour this mixture into the simmering sauce and stir to combine. Bring the sauce to a boil again, then reduce the heat to a simmer. Continue stirring until the sauce thickens to a glossy consistency. Once thickened, turn off the heat and keep the sauce warm while frying the chicken. This step ensures the sauce has the right texture before it is combined with the crispy chicken.
Carefully add the coated chicken pieces to the hot oil in batches, ensuring not to overcrowd the pan. The oil temperature will drop when the chicken is added, so maintain a temperature of 350°F (175°C) while cooking. Stir occasionally and fry for 5 to 6 minutes, until the chicken is golden brown and crispy. Using a slotted spoon, remove the chicken and let it drain on a wire rack over paper towels. Allow the oil to heat back up to 365°F to 375°F (185-190°C) before frying the remaining chicken in batches. This ensures each piece gets evenly crispy and cooked to perfection.
Reheat the sauce over medium heat, adding water if it thickens too much. Once the sauce is hot, add the crispy chicken pieces and toss to coat them evenly. Stir in the remaining sesame oil and orange zest for extra flavor. Serve the coated chicken over a bed of hot rice, garnished with green onions and sesame seeds if desired. This final step ensures the chicken is fully coated in the flavorful sauce and adds a fresh, aromatic finish with the garnish.