Preheat the oven (180 ° C top / bottom heat, or 160 ° C convection). Line a baking sheet with parchment paper.
For the batter, stir the sugar with the butter until frothy. Mix in the eggs one by one. Mix in the flour mixed with the baking powder. Add the remaining ingredients and mix well with a whisk. Smooth out the dough on the baking sheet.
For the quark topping, mix all the ingredients together and carefully spread them on the batter. Then put it in the oven. Bake the tray in the oven at 180 ° C (top / bottom heat) or 160 ° C fan for approx. 25 minutes.
For the topping, drain the mandarins on a sieve. Distribute the mandarins evenly on the cake. Prepare the icing according to the package instructions, but only with 150 ml water and 100 ml mandarin juice. Then distribute on the cake and sprinkle with pistachio nuts.