Place the duck legs in a shallow dish, add the soy sauce, spices and oil. Marinate overnight if possible, or at least 2 hours, turning them over regularly.
Preheat the oven to 170°C.
Halve the plums and remove the stones. Deseed and thinly slice the chilli. Cover the bottom of a large, a heavy-based casserole dish with the plum halves (turned upwards), then sprinkle with the sugar. Add the chilli, cinnamon and star anise.
Place the duck legs on top and pour over the marinade.
Cover with a lid or foil and cook for 2 hours until the meat easily comes away from the bone. Discard the star anise and cinnamon stick, then season to your liking and serve.