Line two baking trays with baking paper. Cream butter, sugar, vanilla sugar, lemon zest and 1 pinch of salt. Stir in the egg. Mix flour and starch and stir in portions briefly. If the dough is too firm, add 1-3 tbsp. cold water and stir in briefly.
Pour the dough in portions into a piping bag with a large open star-shaped spout and squirt onto the baking trays in 6-7 cm long strips. Turn the piping bag tightly over the dough and press the dough out firmly.
Bake in the preheated oven (electric cooker: 175 °C/circulating air: 150 °C/gas: level 2) for approx. 12 minutes. Let them cool down.
Coarsely chop the couverture, melt with white fat in a hot water bath. Dip the biscuits half in each and let them dry on baking paper.