Prepare the plums: If using slightly firm plums, chop them roughly. If using very ripe plums, they will break down easily. Place the prepared plums and the lemon juice into a large, heavy-bottomed pot or Dutch oven.

Macerate and start cooking: Stir the sugar into the plums. Let the mixture sit, stirring occasionally, for about 30 minutes to allow the sugar to draw out the juices (maceration). After macerating, bring the mixture to a boil over high heat, stirring constantly to prevent scorching.

Boil and reduce: Once boiling, reduce the heat slightly to maintain a steady, rolling boil. Add the optional tablespoon of butter to minimize foam. Cook for 30 to 45 minutes, stirring frequently, especially near the end, until the mixture thickens substantially and coats the back of a spoon.

Test for set: To test for a set point, place a small plate in the freezer. Drop a teaspoon of hot jam onto the cold plate. Return it to the freezer for 1 minute. Push the jam with your finger; if the surface wrinkles, it has reached the setting point (approximately 220°F or 104°C).

Jar the jam: Remove the pot from the heat. Skim off any remaining surface foam with a spoon. Carefully ladle the hot jam into sterilized jars, leaving about 1/4 inch headspace. Wipe the rims clean and seal with lids and bands.

Process (Optional for long-term storage): For shelf-stable jam, process filled jars in a boiling water bath canner for 10 minutes, adjusting for altitude. Let cool completely on the counter before checking seals.