Rinse and slice the rhubarb and combine with the jam sugar and cardamom in a pan.
Split the vanilla pod down the middle and then down the middle again and add to the pan. Boil for 5 minutes.
Skim. Remove the vanilla pod and stir the jam quickly, to retain small pieces of the fruit.
Pour the jam into hot, thoroughly cleaned jars. Seal. Store in a cool place.
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