Pick healthy, ripe plums and wash them.
Halve the plums by removing the stones.
Put the plums in a wide cooking pot or a frying pan. If you like the jam to be sweeter, add 1 1/2 cup erythritol. Do not add any water.
Bring the plums to simmer and cover the pot. Cook the plums slowly on low heat and stir with a wooden spoon often, so that the jam doesn't get burned on the bottom.
In the meantime, prepare the jars. Wash them with soap and place them together with the lids in a large cooking pot completely covered with water. Let the jars and the lids boil for at least 5 minutes.
The jam is ready, when a small amount of the jam on a tablespoon or small plate solidifies when cooled. Keep in mind that sugarless jam won't be as thick as the jam with added sugar.
Add 2 tablespoons of cinnamon powder to the simmering jam and cook for another 15 minutes.
Scoop the jam into a jar all the way to the top. Both jam and the jar with the lid should be as hot as possible when filling it. Push the jam down in the jar with a sterilized spoon to remove all the air bubbles and seal the lid. Repeat with the rest of the jam.
Place the jars upside down on a blanket folded in half and cover them. Let them slowly cool for 24 hours.