Wash, halve and stone the plums. Squeeze the lemon.
Put all ingredients in a saucepan, add vanilla to taste (I use a vanilla pod grinder, alternatively the pulp of a vanilla pod can be added) and bring to the boil. Reduce the temperature and simmer for five to ten minutes, stirring occasionally. The plums should be soft.
Finely puree everything with the hand blender until you have a smooth, homogeneous mass. Pour hot into jam jars, close tightly and turn upside down.