Preheat the oven to 175 degrees or with convection 155 degrees.
Grease the edge of a springform pan (diameter 26 cm) and, if necessary, sprinkle with breadcrumbs, line the base with baking paper.
Knead the flour with sugar, salt, baking powder, vanilla sugar, butter and egg. Press the dough firmly on the base and pull up one edge.
Drain the mandarins well, collecting the juice.
Mix the custard powder with 10 tablespoon milk. Bring the rest of the milk with the sugar to the boil and stir in the mixed pudding powder. Bring the pudding to the boil, then stir immediately in a bowl with the sour cream. Spread the cream on the floor. Place the mandarins on top in a circle.
Bake the cake for 60 minutes, then let it rest for 15 minutes in the switched off oven. Then take it out and let it cool down.
Prepare the icing according to the instructions on the packet, but with the mandarin juice and distribute it on the cake.
The best thing to do is to put the cake in the fridge overnight!