Rinse and de-stem the plums in cool water.

Place plums in a thick-bottomed pan suitable for slow cooking.

Add water and cardamom pods.

Bring the mixture to a low boil over medium heat.

Turn heat to low for a slow simmer and cook uncovered for 1 1/2 hours.

Allow the plums to cool.

Strain the cooled plums with a colander, pressing the juice out and collecting it in a large bowl.

Squeeze the plum pulp and skins gently into the bowl, discarding the pits.

Put the plums back into the original pot with the sugar and butter.

Cook at a very low simmer until the mixture begins to thicken, about 4 hours.

Test for adequate pectin development by dropping a spoonful of jam on a plate and refrigerating for a few minutes; it should be soft-set.

Ladle the hot jam into hot, sterile jars.

Wipe the rims clean, place sterile lids on, and tighten the screw caps.

Allow the jars to cool to room temperature and check to be sure that each jar has sealed.