Cut each plum with a knife to open, but do not pit.
Place plums into a large aging container.
Add sugar, vodka and brandy to plums, stirring to combine. Cover or cap container and let age in a cool, dark place for 3 months, stirring monthly. After initial aging, strain liqueur mixture through a wire mesh strainer placed over a large mixing bowl. Press out liqueur in the plums with the back of a wooden spoon. Restrain through cloth until clear.