For the ice cream
Warm together the cream and milk in a heavy saucepan set over a medium heat.
Once the mixture starts to simmer after approximately 5 minutes, remove from the heat.
Whisk together 175 g of the sugar with the egg yolks in a large, heatproof bowl until pale and thick, 2 minutes.
Gradually whisk in the cream mixture in a slow, steady stream until fully incorporated.
Return the mixture to a clean saucepan and cook over a low heat, stirring constantly, until thick enough to coat the back of a spoon, 8-10 minutes.
Strain the custard into a bowl sat inside a larger bowl filled with iced water.
Stir in the vanilla extract and allow the custard to cool, stirring from time to time, for approximately 30-40 minutes.
Meanwhile, combine the cherries with the remaining sugar in a heavy saucepan.
Cook over a medium heat, stirring occasionally, until soft and juicy, 8-10 minutes.
Strain the syrupy liquid from the cherries into a bowl and let it cool to room temperature.
Reserve the cherries for another use.
Stir the cooled syrup into the prepared custard. Pour the cherry custard into an ice cream machine and churn according to the manufacturer's instructions until softly frozen, 20-30 minutes usually.
Spoon the ice cream into a container and freeze for at least 4 hours.
To assemble the ice cream biscuit sandwich
Remove the ice cream from the freezer 5-10 minutes ahead of time.
Place generous scoops of the ice cream on the undersides of four cookies.
Sprinkle the chopped chocolate on top and then sandwich against the remaining cookies before serving.