For the crust: Thoroughly mix all ingredients and press into a springform pan. Bake at 350°F for for 5 minutes. Let cool.
For the filling: Beat the egg yolks with the sugar and vanilla-sugar until pale and foamy. Add the softened butter and beat well, then add the heavy cream and beat again. Add the quark and stir until the mixture is smooth and thoroughly combined.
Beat the egg whites with the salt until very stiff, then very gently fold in the quark mixture, also adding the sifted cornstarch a little at a time. Pour the filling into the crust and smooth top.
Dollop jam on top and using a knife, swirl the jam round. Bake at 300°F for 50 to 60 minutes until well risen and golden. Turn the oven off, and let the cheesecake rest in the oven for 15 minutes, then remove it from the oven and cool for an hour at room temperature (this helps to prevent cracking). Then refrigerate for several hours before releasing sides of pan and serving.