Halve the vanilla pod and mix with the prepared mirabelle plums, the preserving sugar and 100 ml gin. Cover and let the juice steep for several hours.
Then bring it to the boil in a sufficiently large saucepan and let it boil for 4 minutes. Just before the end of the cooking time, stir in the remaining gin.
Pour the finished jam into prepared twist-off jars and leave to cool while standing on the lid.