Beat the fat and sugar with the whisk of the hand mixer until foamy. Gradually stir in the eggs. Add salt, vanilla sugar and lemon zest. Mix flour and baking powder, stir in and gradually add the milk.
Fold blueberries into the dough. Spread on a greased baking tray. Bake in the preheated oven (electric: 175 °C/ gas: level 2) for approx. 35-40 minutes. Let cool on the baking tray. Spread with 1/8 litre advocaat.
Whip the cream until stiff. Add vanillin sugar. Spread on the advocaat. Fill the remaining advocaat into a small transparent bag, cut off a tiny corner and draw the advocaat in strips over the cream.
Decorate with lemon balm. Makes about 20 pieces.