Put the sugar and 150ml water in a pan and bring to the boil. Boil for 1 minute, then remove from the heat and stir through the coconut milk.
Transfer the mixture to a plastic container and allow to cool, then put in the freezer for a minimum of four hours.
Remove from the freezer and turn out onto a chopping board. Cut into small chunks, then blitz in a food processor until smooth and velvety. Serve straight away or keep in the freezer until needed.