The day before, line a sieve with a clean tea towel and place it on a bowl. Stir the yoghurt with sugar and 1 tbsp lemon juice until smooth. Whip the cream until creamy and stiff, while pouring in the vanilla sugar. Fold the cream into the yoghurt. Pour the cream into the sieve lined with the cloth, whip the cloth over it and leave to drain overnight in a cool place.
The next day, sort the blueberries, wash and drain them if necessary. Approx. 1⁄3 Set blueberries aside. Finely puree the remaining berries and the jam with a hand blender. Season with a dash of lemon juice and pass through a sieve. Turn the mousse over onto a plate.
Sprinkle with some blueberry sauce just before serving. Coarsely crumble the biscuits and sprinkle the remaining blueberries over the mousse. Serve immediately. Serve with the rest of the blueberry sauce.