Prune plum butter is really great with cheese-I don't know why, it just is. I have not made this with other plums, but I know from making red plum jelly that plums can be tricky getting them to set. I did not have any trouble with the prune plums, if anything I'd take care not to overcook it. This takes a while, so grab a long handled spoon (it spatters like crazy), a book you can read standing up, and make some of this delicious spread before the plums are all gone. be sure to save the poaching liquid, as it can be turned into a jelly candy by adding an equal volume of sugar, boiling it just past the gelling point, and pouring into a parchment lined pan. Let cool to room temp, then chill uncovered overnight. Next day, cut out shapes and roll in sugar (or coat with chocolate). Stored in the fridge, they last about a month (they won't last that long once people know you made them).
For the Prune Butter:
2 pounds of fresh prune plums (also called, Italian plums)
3 cups granulated sugar
Wash plums, pit and halve them. Place fruit in a large, heavy pot and add 1/2 cup water. Cover, and simmer until fruit is soft. Put through a food mill (a food processor or blender is OK but you must take care not to liquefy it). You should have about 1 1/2 quarts of pulp. If you don't, poach and puree more plums. I always make more than I need as the puree freezes well and can be used out of season as a base for a fluffy pie, or sorbet).
Combine pulp and sugar in a large pot. Cook until it is thick, and can be rounded up on a spoon. Again, you'll want to watch that you don't overcook it. In the last stages, it makes sense to stir it constantly anyway, as it can stick rather horribly. You'll need to reduce the heat as well.
Have sterilised jars ready, along with heated lids and clean bands. If you're unsure about canning procedures, or nervous about water bath canning, this can be stored in the fridge for about a month (possibly longer). For more information on safely canning at home, I like THIS site. Please take the time to read up on the subject before your first attempt. Low acid foods are pretty safe, but you still need to follow some guidelines.
Ladle hot butter into sterilised jars leaving 1/4 inch headspace. Remove air bubbles, wipe tops clean, and seal with a heated lid and screw band. Process 10 minutes in a water bath canner. Makes about 6 half pints.