Preheat oven to 200°C.

Grease and flour a 22-24 cm round cake pan with a removable bottom.

Beat eggs and sugar with an electric mixer until thick and cream-colored.

In a separate bowl, combine flour, baking powder, cocoa powder, vanilla sugar, and salt.

Add the dry ingredients to the egg and sugar mixture, mixing carefully until just combined.

Add quark and lime zest to the batter and stir until well combined.

Pour the batter into the prepared cake pan.

Bake in the preheated oven for 15-20 minutes, or until a toothpick inserted into the center comes out clean.

Let the cake cool completely in the pan before removing it.

Dust the cake with powdered sugar.

Decorate with thin lime slices.

Serve with whipped cream.