Place 2 small plates in the freezer for testing doneness later.

Pit and cut plums into quarters - do not peel.

Zest and juice the lemon and add to plums in a large heavy bottomed, non-reactive pot.

Add all the remaining ingredients to plums, stirring and bringing to a boil. Reduce heat to medium and cook fruit until very tender, about 15 minutes.

Remove and discard the vanilla bean. Ladle about 1/3 of the plum mixture into food processor, and pulse until pureed, but not too much. I like to leave a few very small chunks of plum flesh visible. Continue to puree remaining plums in 2 more batches. Reserve the pureed plums in a separate bowl until all the plum mixture has been pureed. Now return all the pureed plums to the pot and heat over low for about 2 hours, stirring often to prevent sticking. The plum mixture will reduce as is slowly simmers and become thick.

Use the frozen plates to test for doneness by spooning a little plum butter on the plate and tilting the plate. Butter should do a slow slide off the plate, rather than quickly running off.

Remove butter from heat and carefully ladle into sterilized 8 oz, 12oz or pint jars. Leave 1/4" headspace and place lids and rings on jars. (I used 12 oz jars and got 5 full jars of plum butter)

Process in water bath ( I use a steam canner) for 15 min using standard canning procedures.

Let sit for a couple days til set, and store in cool dry location.