Add the plums and jam sugar to a pot. Cook the mixture on medium heat and stir frequently.
Meanwhile, take the cardamom seeds and grind with a mortar and pestle until it's finely ground. Then add the cardamom to the jam.
Once the fruit has softened a bit use a potato masher and mash the fruit.
Add the vanilla and keep stirring the jam often until the water has reduced and the plums are very soft. This should take about 10-15 minutes total, depending on how thick you like your jam or how much water content is in the plums.
The jam is ready when it has thickened. Remove from heat. If you don't like peel in your jam you can use a sieve and strain the peel from the jam, but I didn't think it was needed. All the fruit pieces become so soft.
Store this jam in a clean, tightly sealed container. Enjoy!
Notes
*This qualifies as a ''lightly'' sugared jam, but if you like your jam to be sweeter, you can carefully taste test and add more sugar if needed.