Mix flour, cornstarch, and spices in a shallow bowl. Whisk eggs and water in another.
Double coat each piece of chicken, dredging in flour, then egg, then flour again. Rest on a wire rack for 5 minutes.
Fry in batches in oil heated to 175°C for 4 to 5 minutes until golden. Drain on a wire rack.
Combine orange juice, zest, soy sauce, vinegar, sugar, ginger, and garlic in a saucepan and simmer for 3 to 4 minutes.
Stir in cornstarch slurry and cook until thickened and glossy.
Toss fried chicken in the sauce immediately before serving, garnish with sesame seeds and scallions.