Wash plums thoroughly and place in a large pot with enough water to BARELY cover fruit.
Simmer uncovered, until soft, about 3 hours.
Press through a strainer.
Add sugar and cider and cook for about an hour or until thick.
Seal in hot, sterilized jars and process in a boiling water bath for ten minutes.
**Plum butter thickens by evaporative cooking. If mixture seem thin, there's likely a little too much water in the mixture. Make sure that you're cooking it with the lid off, stir frequently, and continue cooking until it thickens to your preference. You'll be able to speed up the process if you increase the surface area of the plum butter by transferring it to a wider pan, maybe a skillet. Don't increase the heat -- you'll be taking a chance of scorching it.