Put rum liqueur and sultanas in a non-stick saucepan over low heat. Simmer gently until it comes to a very low boil. Take saucepan off the heat and leave aside to cool for 25-30 minutes.
Place butter and chocolate in a food processor and process until it is finely chopped. In the meantime, put cream in a microwave-safe dish. Microwave on medium high for a minute or until it is just hot. Pour the hot cream into a bowl and add the sultana mixture. Leave to cool then refrigerate for 4-5 hours or until the mixture is thick enough to be rolled into balls.
Sift cocoa powder into a shallow dish. Toss the chocolate balls or truffles, one at a time, in the cocoa powder to coat the truffles (make sure your hands are cold). Arrange the truffles on a foil-lined container and dust with remaining cocoa powder. Cover the container and refrigerate until ready to serve.