Inspect 8 half-pint jars for cracks and rings for rust, discarding any defective ones. Immerse in simmering water until jam is ready. Wash new, unused lids and rings in warm soapy water.

Combine plums and water in a large pot over medium-high heat; bring to a boil. Reduce the heat to medium-low, cover, and simmer for 5 minutes. Stir in sugar, then add butter to reduce foaming if needed.

Increase the heat to high and stir constantly until mixture comes to a full, rolling boil. Quickly stir in pectin and return to a full boil. Boil, stirring constantly, for 1 minute. Remove from the heat; skim off and discard any foam.

Pack jam into hot, sterilized jars, filling to within 1/8 inch of the top. Run a clean knife or thin spatula around the insides of the jars to remove any air bubbles. Wipe rims with a moist paper towel to remove any residue. Top with lids and screw rings on tightly.

Place a rack in the bottom of a large stockpot and fill halfway with water. Bring to a boil and lower jars 2 inches apart into the boiling water using a holder. Pour in more boiling water to cover jars by at least 1 inch. Bring to a rolling boil, cover, and process for 10 minutes.

Remove the jars from the stockpot and let rest on a cloth-covered or wood surface, several inches apart, for 12 to 24 hours. Press the center of each lid with a finger to ensure the lid does not move up or down. Remove the rings for storage and store in a cool, dark area.