Grease and flour a 20×25 cm baking pan or a 26 cm springform pan. Preheat the oven to 180 °C top/bottom heat.
Separate the eggs. Beat the egg whites and a pinch of salt until stiff. Slowly add sugar and vanilla sugar. Stir in the yolks. Finally, sift flour with baking powder and fold in. Spread evenly in the pan and bake for about 20 minutes until golden. (Test with a stick). Let cool.
First make the cola jelly. Soak gelatin in cold water for 5 minutes. Pour some of the cola into a saucepan. Squeeze gelatin, heat cola and dissolve gelatin in it. Do not boil. Let cool slightly and mix with remaining cola gradually. Squeeze half a lemon and add to the cola. Add a little sugar to taste.
Cover the baking pan of the sponge cake with plastic wrap and pour the cola mixture into it. Allow to set in the refrigerator.
Mix room temperature curd cheese and mascarpone. Stir in lemon juice from 2-3 lemons (according to taste) and sugar. Stir in first a part of the whipped cream,then the rest together with the cream stiffener, but not too long.
Spread the cream on the sponge cake, possibly make a wave pattern with a fork (you can see through the cola jelly). Turn the jelly out onto a board and then turn it out again onto the cake. Serve well chilled.