In an ideal world, sterilise the jars and any other equipment before you start. Put a couple of saucers in the freezer, for testing whether the jam is ready later on (or use a sugar thermometer). Put the plums in a (preserving) pan and add 200ml water. Bring to a simmer, and cook for about 10 mins until the plums are tender but not falling apart. Add the sugar and lemon juice, then let the sugar dissolve slowly, without boiling. This will take about 10 mins.
Increase the heat and bring the jam to a full rolling boil. After about 5 mins or more, spoon a little jam onto a cold saucer. Wait a few seconds, then push the jam with your fingertip. If it wrinkles, the jam is ready. If not, cook for a few mins more and test again, with another cold saucer. If you have a sugar thermometer, it will read 105C when ready.
Take the jam off the heat and add (if using) the cinnamon sticks and the knob of butter. Let the jam cool for 15 mins. Ladle into hot jars, seal. Will keep for 1 year in a cool, dark place.
2 kilo plum / 1.5 kilo sugar / optional 200 ml water / optional lemon / optional knob of butter to clear “scum”
Tip: Wash plums, then take a round handle wooden spoon and push through from join to tree, then you shall have pitted the plum yay