Wash the plums and cut them into small pieces (1¼ cm or so), removing the stone as you do it. Place them in a heavy pan and pour a little water in the bottom, no more that a half a cup.
Bring the plums to a boil, stirring often so that they don't burn on the bottom. Add the cinnamon stick and the cloves.
Boil gently for 3 or more hours, stirring often.
Place the cooked plums in a ricer or sieve and stir to remove the skins and spices. Bring the jam/butter back to a boil until it has reduced as far as you want it and is very thick. Ladle the butter into clean, boiled, preserving jars or into freezer bags. Freeze or can, as you wish.