Cut the plums in half and discard the pits. Add the plums to a 1 litre canning jar. Top with the sugar and add the brandy and vodka. Make sure there is enough brandy to cover the fruit. Screw a lid on the jar and shake it gently a few times to distribute the liquid around the fruit and sugar.
Let the brandy steep for 3 weeks in a cool place. Each day for the first few days, flip the jar back and forth a few times to encourage the sugar to dissolve. Once the sugar has dissolved, you can give the jar a gentle shake every couple of days until it has finished steeping.
After steeping, strain the brandy and discard the fruit. Store the brandy in the fridge where it will keep for a few months. The plum brandy can be served chilled as a digestif after a meal or used in place of regular brandy. Enjoy!