Put the blackcurrants in a processor and whizz until smooth and even the skins have turned to purée.
In a pan, dissolve the sugar into 200ml (7fl oz) of water, bring to the boil and simmer for 2 minutes. Remove and cool.
Mix with the purée and put in the freezer. Then mix every 15 minutes for 2 hours, and then every half an hour until the granita appears grainy and all the crystals have broken - it'll look like snow or coarse sand.
Some products in this recipe are seasonal and so may not be available to buy online all year.