Combine vinegar, sugar, garlic, chilli and star anise in a small saucepan stir over low heat for 4 minutes or until sugar has dissolved. Bring to a boil. Reduce heat to medium. Simmer for 5 minutes or until reduced by one-third and slightly thickened. Remove the flavourings and discard. Set aside.
Season the duck breasts well with salt and pepper.
Heat the oil in a frying pan over medium high heat and place the duck breasts in skin-side down. Cook 6 minutes until crispy, then turn over and cook a further 4 or until done to your liking. It's normal to serve duck breast cooked medium to rare. Rest for 5 minutes before serving
While the duck is resting, pour the boiling stock over the couscous and set aside covered for 5 minutes. Stir the butter through with a fork fluffing up well.
Place the plums in the reduction and reheat gently.
Slice duck breasts and serve them and the plums over couscous.