Combine the rice wine, cornflour, 2 Tbsp dark soy sauce and 2 tsp sesame oil in a large non-metallic bowl. Toss the chicken, cover and marinate in the fridge for 1 hour.
Meanwhile, soak the citrus peel in warm water for 20 minutes and then chop. You want about 1 1/2 tsp of chopped peel.
Heat the oil in a wok over high heat. Drain the chicken from the marinade using a slotted spoon and stir-fry in batches for 2 minutes or until browned and just cooked through. Remove from oil and leave to drain.
Drain all the oil except 1 Tbsp and reheat then add the chilli flakes and ginger stir-frying for 10 seconds. return the chicken to the wok and add the spring onion, sugar, soaked citrus peel, soy sauce and sesame oil. Add 1/2 tsp salt if desired. Stir-fry for 2-3 minutes or until well combined and warmed through.
Garnish with spring onion and serve.