Preheat the oven to 180 °. Grease either a 22 cake form or a small Guglhupf form. Line the bottom of the cake tin with baking paper.
Mix the butter, sugar and vanilla sugar until frothy. Gradually stir in the eggs while adding the juice and zest. When everything has become creamy, add the mascarpone and whip vigorously. Mix flour with baking powder, gradually sift into the butter-mascarpone mixture, stir in and pour the batter into the mold.
Slide into the oven from below on the 2 runners and bake for 40 minutes. Then switch the baking temperature down to 160 ° and bake for another 10 minutes. Then do the wooden stick sample. No dough should stick to it when it is pulled out. But if this is still the case, extend the baking time by 5 minutes and keep checking with the chopsticks. When the cake is baked through and the stick can be pulled out smoothly, switch off the oven and let the cake stay in it for another 10 minutes.
Then take it out and let it cool down in the pan. In the meantime, mix the icing with the sifted powdered sugar and a little lemon juice. But gradually add the juice with a tablespoon (approx. 3 - 4 tablespoons), so you can better keep an eye on the consistency. The cast should not be too fluid, but also not too compact.
Turn the cake, which is still very slightly lukewarm, out of the mold onto a cake plate and coat it thickly with the icing. It tastes slightly lukewarm / if you can't wait ... ;-), but also completely cooled down, has a consistency like marzipan, melts on the tongue ...... and caresses the hips ...... which hips .. ;-)))))))