Pour the mixture into a freezer container and freeze until set, about 3 hours. Remove from the container and chop roughly into 7.5 cm/3in pieces. Place in the bowl of a food processor and process until smooth. Return to the freezer container, and freezer again until firm. Repeat the freezing-chopping process 2 or 3 times, until a smooth consistency is reached, then freeze until required. Alternatively, use an ice cream maker and freeze the mixture following the manufacturer’s instructions.