Preheat the oven to 200 ° C (top / bottom heat, hot air: 180 ° C). Grease the tart pan. Drain the mandarins in a colander.
For the dough, beat the eggs, sugar, vanilla sugar and oil until frothy. Mix the flour and baking powder and slowly stir in alternately with the buttermilk until a smooth dough is formed. Pour the batter into the tart pan. Spread the drained mandarins on the dough.
Put the tin on the wire rack in the preheated oven. After approx. 30 - 35 minutes, take it out of the oven and let the tart cool in the tin.
Prepare an icing from the cake glaze, sugar and nectar according to the instructions on the packet. Spread the icing on the tart from the inside out with a spoon.
If you don`t have or don`t like lemon buttermilk, you can also use pure buttermilk and add about 1 teaspoon lemon zest to the batter.