Chop the plums roughly, and place them in a large pot. Cook over medium low heat for about an hour, stirring occasionally to prevent plums sticking to the bottom of the pot.
Add sugar and cook for additional 30 minutes, stirring occasionally. Take the pot of the heat.
Using hand blender, carefully (it may splash a little) process the hot mixture until smooth.
Place the pot back on the stove. Bring the mixture to a boil, add chopped chocolate, vanilla and coffee liqueur. Cook for few minutes until smooth and well combined.
Place the plum-chocolate mixture in sterilized jars. Cover the jars and screw on the lids until they're snug but not overly tight. Screwing a mason jar lid too tight will actually create a weaker seal, as it will damage the threads in the lid.
Process the jars in a boiling water bath according to manufacturer instructions.
Take the jars out of the bath and let them cool at room temperature.
Store the jars in a cool and dark place, or in the fridge. Well sealed jars will keep for few months.