Testing - Damson Cheese, or Plum Paste or Pumbrillo

https://www.saveur.com/plum-paste-plumbrillo-recipe/
http://experimentalmouffette.blogspot.com/2017/12/testing-damson-plum-cheese.html

3½ lb. ripe plums
12 oz. (1½ cups) sugar, plus more as needed
Butter
Optional - 3 Tbsp. fresh lemon juice

1. Wash the plums (do not peel or remove the pits) and place in a large, heavy-bottomed pot. Add enough water to barely cover the plums, then bring to a boil over medium-high heat. Lower the heat to maintain a strong simmer, and cook until the fruit is very tender, 20–25 minutes. 
2. Remove from the heat, and let cool slightly. Squeeze out the pits and put the emptied, cooked fruit through a fruit masher and sieve (pictured). 
3. In a large strainer or in a muslin bag set over a large, heat-resistant bowl, let the plums strain out their excess juices (do not press on the fruit or attempt to force the pulp through the strainer). Reserve the plum juice for another use. 
4. Measure the pulp (you should have about 2½ cups or 1¼ pounds). Stir in the sugar, adding more as needed to maintain a ratio of 6 parts sugar to 10 parts fruit. Add the lemon juice, cover, and refrigerate for at least 8 hours and up to 1 day.
5. Meanwhile, put a small plate in the freezer (to test the mash, later).
6. Bring to a boil then cook the mash at the lowest setting on the stove top until it gets so thick that when you scrape a wooden spoon along the bottom it stays apart for a few seconds before it to close up again. Make sure to stir frequently. This will take a long time, over an hour.
7. NOTE: If you're using a candy thermometer, the mixture should never exceed 221F)
8. Retrieve the plate from the freezer and place a small drop of the plum mixture on the surface. If it starts to set and form a skin after a minute, the preserves are ready. If it remains runny, return the plate to the freezer and cook the plum mixture a few minutes more before testing again.
9. Boiled some glass jars along with their lids and when they are still warm slathered a tiny bit of butter inside before putting in the damson mush.
10. Remove the plum mixture from the heat. Ladle carefully into sterilized jars leaving 1/2" space at the top, then clean the rims and sides with a kitchen towel.
11. Seal firmly with the lids, then loosen by 1/4 turn. Check for air bubbles, and if necessary, tap the jars gently on the towel to work any bubbles upwards and out. 
12. For water bath canning, put the jars on a rack in a large pot of boiling water. The jars must not sit directly on the kettle bottom. Adding the jars with slow the boil. Return the water to a good boil and only then start to count the processing time: for 235ml or smaller jars, 10 minutes. For larger jars, 20 minutes.
13. In a draft-free spot where the jars won't be moved, place a towel over a baking sheet. Leave to cool for 12 - 24 hours to fully seal the jars, and allow the cheese to set.
14. OPTION: if for immediate use or if you don't want the bother of canning, let cool completely and refrigerate for up to 2 weeks or freeze for up to 3 months.