Grease a round (26 cm Ø) or rectangular (approx. 20 x 28 cm) tart mould with a removable base and dust with flour.
Cream butter, sugar, vanilla sugar and 1 pinch of salt with the hand mixer. Stir in the eggs one by one. Mix flour, starch and baking powder and stir in 2-3 tbsp. lemon juice and lemon curd.
Paint into the mould. Bake in the preheated oven (electric oven: 175 °C/circulating air: 150 °C/gas: level 2) for approx. 25 minutes. Let cool down for about 15 minutes.
Stir icing sugar and 1-2 tablespoons lemon juice until smooth. Pour into a freezer bag and cut off a small corner at the bottom. Decorate the cake with it in strips and let it dry.