Fill a large pot with a rack halfway full with water and bring to a boil.

Put chopped plums into a heavy sauce pan and mash with a potato masher.

Stir in sugar, spices, and add butter.

Bring to a full rolling boil. Stir in pectin, returning to a full rolling boil, and continue to boil for exactly one minute, stirring constantly. Remove from heat, and skim off any foam.

Ladle into clean jars. Wipe jar rims and threads, and cover with lids. Screw bands until finger tight.

Put jars into pot with boiling water, on a rack. Make sure jars are covered in water. Process in gently boiling water for 10 minutes.

Remove from water and allow jars to cool on a towel. Lids are sealed if the center does not spring back after pressing the center. (You'll hear the pops when the lids seal themselves.) If not sealed, refrigerate.