Set several small plates in the freezer. They'll be used to test the consistency of the butter later on.

Place the plum halves in a medium saucepan along with any of the juices that may have run out onto the cutting board. Cook over medium heat until the fruit is very soft (10 - 15 minutes). Transfer to a blender, and carefully blend until pureed (I used the lowest setting on my Vitamix). Measure out 2 cups of plum puree, and reserve the rest for another use (I add to chia pudding!).

Return the plum puree to the medium saucepan along with the sugar, honey, lemon juice and pinch of kosher salt, stirring to combine. Bring the mixture to a boil over medium-high heat, reduce the heat to medium-low, then cook until sufficiently thickened (18 - 20 minutes).

To test the consistency of the butter, place a small spoonful of the hot butter on a cold plate, and return to the freezer for 1 minute. If the mixture looks thin and runny, it's not done. If it's thick and holds it's shape it's done.

Transfer Plum Butter to a clean jar, and let cool before storing in the refrigerator for up to 1 month.