I often freeze a bag of pitted plums to use in cocktails like this as well as baking. They hold their shape well when frozen and have a great flavour.
Halve 6 plums, remove pits and place on a tray to freeze for at least 4 hours. (Frozen plums can be bagged for easy free- flow use. They will keep for months.) While plums are freezing, heat ½ cup sugar and ½ cup water, stirring until dissolved. Simmer for 5 minutes. Chill. To serve, blend frozen plums and syrup with ¼ cup lime juice until slushy. Add 150–200mls white rum or vodka and blend again to combine. Divide between 4– 6 glasses and serve at once.