Bring the blueberries to the boil with sugar, lemon and water and simmer for about 2 minutes, stir frequently, the berries should be mushy (mushy), stir in rum and let cool.
Mix the mascarpone with yoghurt until smooth, sweeten with powdered sugar if you like, roughly crumble the biscuits and stir in, distribute between dessert glasses.
Pour the cooled berry puree over the cream, decorate with whipped cream and a few blueberries, chill