For plum butter, it’s less a recipe than a process. If you’ve made apple butter, the process is similar
I pit and split the plums, then cook on low with just enough liquid* to keep them from scorching until they’re soft enough to be pureed with the stick blender. Then I add say a cup of sugar to a pound of plums and cook very low, stirring to keep from scorching, until a spoonful stands up on a plate with no liquid seeping.
A slow cooker w/o the lid on works for this, but you still have to stir
I don’t add spices, want it to taste like plums
I water-bath can this, so I can’t speak to its shelf life without
*If you can the plums [I recommend] the canning liquid is good for this