In a large bowl, beat the egg yolks, sugar and vanilla-sugar until pale and foamy. Add the butter and beat until incorporated, then add the the heavy cream, beating until mixed well. Finally, add the Quark and stir until the filling is smooth.
In a small bowl, beat the eggs and pinch of salt at high speed until stiff peaks form. Sprinkle the cornstarch over the top of the cheesecake filling, then gently fold the egg whites into the quark mixture until all fully combined.
Pour the filling into the prepared crust, shaking the pan gently until the surface is smooth and all air bubbles are released. Trim the top of the dough to within one inch of the filling.
Bake for 50-60 minutes, until well risen and golden. It will rise like a souffle, but sink upon being moved. This is normal.
After the cheesecake is golden, turn the oven off, leaving the cheesecake in it for 15 minutes. Then remove the cheesecake to a wire rack to cool for an hour.
Refrigerate for several hours before releasing the sides of the pan and serving.
It is best if made a day ahead. Keep the cheesecake covered, in the refrigerator.