For the base, whisk eggs, 300 g sugar, vanilla sugar and salt with the whisks of the mixer until creamy. First stir in flour and baking powder, then oil and lemonade. Spread the dough on the greased baking tray (approx. 35 x 40 cm) until smooth. Bake in the preheated oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer) for approx. 20 minutes. Let them cool down.
Drain the mandarins, collect the juice and possibly top up with water at 3⁄4 l. Halve the base lengthwise to make 2 smaller plates. Form two rails from aluminium foil folded several times (each approx. 8 cm high and approx. 80 cm long). Lay the rails around the floors and close them with paper clips, for example. Spread mandarins on the floors.
Mix casting powder and 5 tablespoons of sugar in a pot. Stir in the juice bit by bit with a spoon. Bring to the boil while stirring. Spread the icing on the mandarins (thanks to strips of multi-folded aluminium foil, the icing does not run down the sides of the cake) and chill for approx. 45 minutes.
Whip cream in 2 portions until stiff. Whip the sour cream until stiff, allowing the jam sugar to trickle in. Fold in the cream. Pour cream on the mandarins and spread carefully. Refrigerate for another 1 1⁄2 hours. Mix 3 tbsp. sugar and cinnamon and sprinkle over the cake.