To make the semi freddo, add the egg yolks, sugar and vanilla to one large bowl, the egg whites to another and the cream to a third slightly smaller bowl. Whisk the egg whites with a handheld electric mixer until they form stiff peaks. Using the still dirty whisk, whisk the egg yolks and sugar together for 4-5 minutes until thick and pale. Rinse the whisk beaters, dry and then softly whip the cream.
Fold the cream into the egg yolk mix then the crushed biscuits and finally the whisked egg whites. Spoon the mixture into a large metal loaf tin, cover the top loosely with clingfilm then freeze the mixture for 3-4 hours or until firm enough to scoop.
About 30 minutes before serving, add the granulated sugar and cold water to a frying pan for the caramelised cherries, heat gently until the sugar has dissolved, stirring very occasionally. Increase the heat and boil for 3-5 minutes, without stirring until the syrup is golden.
Take the pan off the heat, add 2 tablespoons boiling water and stand well back as the hot syrup will spit. When the bubbles have subsided slightly tilt the pan to mix the extra water into the syrup. Drizzle 6 tablespoons of the hot caramel over a baking tray lined with nonstick baking paper.
Quickly add the 4 more tablespoons of boiling water to the syrup still in the pan then the cherries, heat together for 3-4 minutes until the syrup has remelted and the cherries have softened and coloured the syrup. Set aside until ready to serve. Transfer caramel on the paper to the fridge to harden.
Scoop the just set semi freddo into bowls, reheat the caramelised cherries, if needed, spoon over semi freddo and decorate with the chilled caramel, broken into shards. Serve immediately!
short of time? Serve the caramelised cherries over scoops of good quality readymade vanilla ice cream.