In a bowl, alternate layers of plums with sugar.
Add lemon juice.
Cover with a cloth and let macerate for 12 hours in the refrigerator.

Pour the contents of the bowl into the jam pan.
Bring to a boil for 5 minutes.
Then remove the fruits with a skimmer and pass them through a vegetable mill (medium grid) or a food mill.

Pour the preparation into the jam pan and add the coconut milk.
Mix well and bring to a gentle simmer for 15 minutes, carefully skimming.
Continue cooking for 10 to 15 minutes, until the jam has a thick consistency or until the candy thermometer reads 220 F (104 C).
You can also check the cooking by pouring a little jam onto a cold, tilted plate: it should set instantly and not run.

Put immediately into hot, dry jars.
Cover while hot and store in a dry, cool place away from light and moisture.