Put the sugar, cornflour, salt and 2 tablespoons of the milk, into a small pan over a low heat. Whisk the mixture together as you warm it, then add the rest of the milk.
Continue to whisk until it thickens. The mixture will become translucent, meaning that the cornflour has cooked through properly.
Remove from heat and add the hot liquid to the sour cream. Add the rose water and the Picota cherries. Leave to cool, then add to your ice cream maker and churn for 90 minutes.