Wash plums, and slit each in 3 places, using a paring knife. Combine plums and water in a Oat-bottomed kettle. Bring to a boil; boil 30 minutes, stirring frequently. Remove from heat, and press through a sieve. Discard skins and seeds.
Return pulp to kettle, and add remaining ingredients, stirring well. Cook over medium-low heat, stirring constantly, 20 minutes or until mixture thickens.
Quickly ladle butter into hot sterilized jars, leaving 1/4-inch headspace. Cover at once with metal lids, and screw bands tight. Process in boiling-water bath 5 minutes.