In a medium bowl, combine the chicken pieces with soy sauce, rice vinegar, sesame oil, cornstarch, and beaten egg, then mix well and let it marinate for at least 15 minutes, or up to 30 minutes in the refrigerator.

In a separate bowl, whisk together the orange juice, soy sauce, rice vinegar, brown sugar, minced garlic, grated ginger, and red pepper flakes (if using), and in a small bowl, dissolve cornstarch in water to create a slurry.

Heat vegetable oil in a large skillet or wok over medium-high heat until it reaches 350-375°F (175-190°C), or test with a small piece of chicken.

Carefully add the marinated chicken to the hot oil in batches, making sure not to overcrowd the pan, and fry for 3-4 minutes per side until golden brown and cooked through, then remove the chicken with a slotted spoon and place it on a wire rack to drain excess oil, repeating with the remaining chicken.

Carefully pour out most of the oil from the skillet, leaving about 1 tablespoon, and return the skillet to medium heat, then pour in the orange sauce mixture and bring it to a simmer, stirring constantly.

Once simmering, stir in the cornstarch slurry and cook for another 1-2 minutes, or until the sauce thickens to your desired consistency.

Add the fried chicken to the thickened orange sauce and toss to coat evenly, ensuring all pieces are well-covered.

Serve the orange chicken immediately with steamed rice and a sprinkle of sesame seeds or chopped green onions for garnish, if desired.