Place the butter in a bowl and stir in the sugar, vanilla sugar and salt. Mix in the egg yolks and continue to stir until the mixture becomes lighter in colour. Combine the flour, baking powder and lemon zest, add to the mixture, cover and refrigerate for approx. 1 hr. Chill one quarter of the pastry for the crumble topping. Roll out half of the remaining pastry on to the prepared cake tin base, close the tin frame. Shape the remainder of the pastry into a roll (approx. 60 cm long), place around the edge of the tin, press gently and pull upwards approx. 4 cm. Prick the base firmly with a fork, refrigerate for approx. 15 mins.
Spread the jam over the pastry base. Mix the quark with all the other ingredients up to and including the salt, spread over the pastry base.
Mix the plums with the sugar, spread on top of the quark mixture. Using your hands, crumble the chilled pastry and scatter over the plums.
Approx. 1¼ hrs. in the lower half of an oven preheated to 180°C. Remove, allow to cool a little, remove the tin frame, slide the cake onto a cooling rack, leave to cool and dust with icing sugar.