In a medium bowl, whisk together the cornstarch, flour, egg, and 2 tablespoons of water to create a thick batter.

Add the chicken pieces to the batter and toss to ensure each piece is evenly coated.

Heat the vegetable oil in a large pot or Dutch oven over medium-high heat until it reaches 350°F (175°C).

Carefully add the battered chicken pieces to the hot oil in batches, being careful not to overcrowd the pot; fry for 3-4 minutes per batch until golden brown and cooked through, then remove with a slotted spoon and place on a wire rack to drain.

In a saucepan, combine the orange juice, rice vinegar, soy sauce, brown sugar, grated ginger, minced garlic, orange zest, and red pepper flakes.

Bring the mixture to a simmer over medium heat, then in a small bowl, whisk together 1 tablespoon of cornstarch and 2 tablespoons of water to create a slurry.

Slowly whisk the cornstarch slurry into the simmering sauce, stirring constantly until the sauce thickens to your desired consistency.

Remove the sauce from heat and set aside.

In a large bowl, gently toss the fried chicken with the prepared orange sauce until all pieces are well coated.

Garnish with sliced green onions and sesame seeds if desired, and serve immediately with steamed rice or noodles.