Wash and cut the plums in half. Remove the pit and chop each half in quarters.
Add plums, sugar, lemon juice and cinnamon stick to a medium-sized saucepan.
Cook over high heat stirring to dissolve the sugar. Bring the mixture to a boil, stirring often to avoid burning the fruit.
Lower heat to medium-high and let the jam boil for about 12 minutes, stirring frequently until it thickens. If you don't like pieces of fruit in your jam, use a potato masher or a fork to mash the fruit.
Check if the jam is ready by making a streak with your finger down the back of jam covered spoon. If the gap stays set then you know the jam is ready. If the jam is still runny then boil it for a few minutes longer and check again.
Once the jam has thickened to your liking remove it from the heat. Let the jam cool a bit before transferring into a clean jar.
Let the jam cool completely, seal the jar and refrigerate for up to 3 weeks.
One serving equals two tablespoons of jam. Two tablespoons contain approximately 100 calories.
If you are watching your sugar intake you could use a little less sugar than stated in the recipe. Taste the jam while you are making it and if it's not sweet enough you could add a bit more sugar.
If you prefer a plum jam without pieces of the fruit's skin, you could use a fork to remove the plum skins from the jam once it's cooked. Or you could chop plums finely in a food processor before cooking them. Don't peel the plums as its skin contributes to the colour and flavour of the jam.