In a bowl, combine chicken pieces with soy sauce, rice vinegar, grated ginger, and minced garlic; marinate for at least 15 minutes.

Whisk together flour, cornstarch, baking powder, egg, and water in a separate bowl until smooth.

Dip each marinated chicken piece into the prepared batter, ensuring it's fully coated.

Heat vegetable oil in a large pot or deep skillet to 350°F (175°C), then carefully fry battered chicken in batches until golden brown and cooked through, about 3-4 minutes per batch. Remove and drain on a wire rack.

In a saucepan, combine orange juice, orange zest, soy sauce, rice vinegar, sugar, ginger, garlic, and red pepper flakes (if using); bring to a simmer.

Whisk cornstarch and water to create a slurry, then gradually add it to the simmering sauce, stirring constantly until the sauce thickens.

Add the fried chicken to the thickened orange sauce and toss to coat evenly, then garnish with green onions and sesame seeds if desired before serving immediately.