Line a 30 x 20cm tray with cling film. Put the plums in a pan with 2 tbsp water and 110g of the sugar. Cover with a lid and stew over a medium heat for 10 mins until soft. Allow to cool slightly, then blitz with a hand blender. Pass through a sieve to remove the plum skins. Allow to cool.
Put the remaining sugar in a pan with 150ml water, and dissolve over a low heat. Boil for 5 mins or until the mixture reaches 120C on a cooking thermometer.
To make the meringue base, whisk the egg whites until stiff. With the beaters running, carefully pour the sugar mixture onto the egg white, whisking until thick. In another bowl, softly whip the cream, then gently fold in the liqueur, plum pulp, biscuits and the meringue. Pour into the tin, cover with cling film and freeze for a few hrs until set. To serve, remove from the freezer 5 mins before scooping into balls.