Puree the yogurt with the sugar and 3 finely chopped pineapple slices. Squeeze the mandarins and add the juice to the yogurt mixture. Stir in the low-fat quark. Whip the whipped cream with the vanilla sugar until stiff and fold into the quark cream. Cut the remaining 3 pineapple slices into small pieces and fold in.
Now put some of the cream in a mold or bowl. Place a few speculoos on top and smooth the rest of the cream on top. Put some speculoos on top.
When using ladyfingers, layer the cream in 3 parts with 2 layers of ladyfingers and put some pineapple pieces on top as a decoration.
Let it soak in the refrigerator for a few hours. Dust with powdered sugar if necessary.