Wash the plums, quarter them and take out the stone. Mix with the brown sugar, the cinnamon sticks, the vanilla seeds and the star anise. Put into a deep roasting tin and spread evenly. Cook on the bottom shelf of a pre-heated oven at 250 degrees centigrade for about 25 minutes. Cover the bottom of the oven with tin foil in case fruit juice boils over.
Reduce temperature to 180 degrees and leave to simmer for 3½ to 4 hours. Leave the oven door a little bit open so the steam can escape. Keep the jam simmering at all times and stir occassionally. Fill the hot jam immediately into clean, sterilised jars. Screw the lids on tighlty and put upside down for 5 minutes.