Place plums and water into a large pot, and bring to a boil. Reduce heat to medium-low, cover, and simmer for 5 minutes. Stir in sugar little at a time ( make sure all sugar dissolves), and add butter to reduce foaming, if needed. Bring mixture to a full rolling boil over high heat, stirring constantly, and then mix in pectin quickly (stir until dissolved). Return jam to a full boil for 1 minute, stirring constantly. Remove from heat, skim off and discard any foam.

Sterilize the jars and lids (do this prior to starting jam) for 5 minutes. Pack plum jam into jars, filling the jars within 1/8 inch of the top. Run knife or thin spatula around inside of jars to remove any air bubbles. Wipe the rims of jars to remove any jam residue. Top with lids and screw with rings.

Place filled jars in pot of boiling water (cover water 1 inch above jars). Bring to a full boil over high heat for 10 minutes.

Remove jars from water, place onto a towel to cool. Once cool press top of each lid to ensure that the seal is tight. Store in fridge