Trim the membrane from the rack of lamb, and rub with salt and rosemary leaves. Halve the plums and remove the stones. Add the wine and the anise to the lamb broth, bring to the boil and then strain. Bring back to the boil and then add the plums. Simmer for 5 minutes, then cover and put to one side. Meanwhile, add the lamb to a frying pan and brown on all sides in a little oil. Pre-heat the oven to 160 degrees Celsius and roast for a further 10 minutes. Reheat the plum sauce before serving and stir in the cold diced butter.