Shortcrust pastry: Quickly knead all ingredients for shortcrust pastry except cornstarch and flour until smooth. Work flour and cornstarch underneath. Rest the dough in the refrigerator for 10 minutes. Now roll out, cut out a ring of about 28 cm and place on a baking sheet lined with parchment paper. (Leftover dough can be frozen without any problem.) Prick the dough a few times with a fork and then bake in a hot oven at 180 °C for 10 min.
Sponge cake: melt the butter in a saucepan and let it boil once, then remove it from the stove. Beat sugar, eggs, vanilla sugar, a little salt and lemon peel in a water bath until warm. Then remove the baking bowl from the water bath and continue beating for 6-8 minutes until cooled. Sift flour and cornstarch on top and fold in. Finally, fold in the melted warm butter. Fill the amount into a 28 cake springform pan or a 28 ring cake ring placed on a baking sheet lined with parchment paper. Bake in the oven set to 180 degrees (convection) for 2 minutes, then turn down to 165 degrees and continue baking for about 18 minutes.
Cool the base well. Cut a 1.5 cm thick slice from the sponge cake base. (The rest of the base can be frozen without any problems). Cut the base smaller with a 26 springform pan rim.
Yogurt mousse: whip cream until creamy and soak 5 sheets of gelatin in cold water. Mix yogurt with powdered sugar until smooth. Pour some yogurt into a saucepan.