The day before, soak 4 wooden skewers in water for 20 mins. Wash the ducks inside and out with cold water, drain and pat dry with kitchen paper. Mix together the sugar, star anise, ginger, spring onions and a few good pinches of salt, then use this to fill the cavities. Close the cavities with wooden skewers and set aside.
In a small bowl, mix the maltose and 2 tbsp of the vinegar. Add the remaining vinegar to a jug of boiling water and pour over the ducks. (The boiling water opens up the pores, while the vinegar helps to strip some of the waxiness from the skin, so it will be more receptive to the maltose, which adds sweetness and a lacquered caramel colour.) Smear the maltose mixture over the ducks, then place them in a large plastic container and put in the fridge overnight, uncovered.
To cook, heat oven to 220C/200C fan/ gas 7. (Fan ovens are particularly effective for roasting duck.) Put a little water in the bottom of a large roasting tin, place the ducks on a rack over the top and cover with foil. Roast for about 45 mins. Take off the foil and roast for another 45 mins – the duck must be well done, there is no such thing as a rare Chinese roast duck! Take the duck out of the oven and let it rest for a good 20 mins before carving.