Wash, stone and quarter the plums. Weigh 1 kg of flesh. Mix jam sugar and plums well in a high pot and let it soak for 3-4 hours. Bring the plum-sugar mixture to the boil while stirring, then boil it for exactly 4 minutes until it becomes bubbly.
Stir in Grappa. Immediately fill jam to the brim into clean glasses. Close them with twist-off lids and let them stand upside down for about 15 minutes. Then let it cool down and store it as cool and dark as possible.
Shelf life approx. 12 months