Summary: A lovely, flavorful jam that pairs wonderfully with whole wheat-molasses bread.
Watch the pectin levels here. Plums, especially that Italian prune plum variety, have high pectin levels so added pectin can make for an extremely stiff jam. Start with a lesser amount of pectin (in my experience). If your jam ends up too thick, add boiling water a couple tablespoons at a time until you reached the desired consistency. Go looser than you think because after processing your jars, the jam will tighten up again.