Plum butter is really just another way of saying Plum jam, but it isn’t made with pectin but simply by boiling the plums with sugar. Apple butter is made in the same way. It is very easy to burn the butter, so needs to be stirred often and cooked over low heat. Plum butter is better when the plums are ground in a processor or mill sometime during the cooking. I used about 2 parts plums to 1 part sugar. Adding a little water to get it started helps. You don’t need to be precise, but decide how sweet you like it. The sugar helps thicken the jam.
I read that if you freeze some plates to use for putting a dab on to test for doneness, it helps. It should be almost as thick as jam, I think, but you can choose how thick you like it.
I always added some coarsely ground walnuts to the plum butter, I think it makes both taste better. Adding a little lemon juice is also helpful.
I like the look and taste of jams made with pectin better. The long slow cooking turns the fruit rather dark.