Combine plums, apples and juice in a heavy non-aluminum saucepan. Bring to a boil over medium heat. Reduce heat, cover partially and simmer 25 minutes, stirring occasionally. Uncover and cook until fruit is very tender, stirring stirring occasionally (20-30 minutes).
Cool slightly. Force fruit through food mill fitted with medium blade, discarding peel and seeds. (or you can do as I do, seed and pit the fruit BEFORE cooking. Then, when you reach this stage, all you have to do is run it through a food processor) Return puree to pan, add sugar and cinnamon. Bring to a boil over medium heat, stirring constantly. (careful, mixture can scorch easily!) Reduce heat and simmer briskly until butter is thick and glossy, stirring frequently (about 40 minutes). Remove from heat, spoon into hot sterilized jars and seal. Makes 3 pints jars or 6 - 8oz jars. Butter can be stored in the fridge for up to 3 weeks, or processed in a boiling water bath (15 minutes for pint jars or 10 minutes for 8 oz jars) and kept on a cool dark shelf for up to 1 year.