Tip the cherries, lemon zest and juice, the sugar and cornflour into a pan. Cook over a medium heat for 12-15 mins until bubbling, reduced and thickened. As the cherries cook, lightly mash using a wooden spoon to break them up. Cool completely.

Whisk the cream and pistachio butter to soft peaks using an electric whisk, then fold in the condensed milk. Pour into a 1.2-litre loaf tin or freezerproof container, then swirl in the cooled cherries using a cutlery knife. Scatter over the pistachios, cover and freeze for at least 6 hrs, or ideally overnight. Will keep frozen for three months. Remove from the freezer for 5 mins before scooping.