First, wash the plums, remove the seeds, cut in half. Heat in a saucepan and then puree with a hand blender.
Then strain through a coarse sieve to remove any remaining coarse pieces. Return to a saucepan and heat again with the preserving sugar.
Season with cinnamon and pour the plum jam into jars while still hot.
Close jars and place upside down on a wet cloth.
Allow to cool.