Sift the flour, stir in the baking powder, salt, vanilla-sugar, lemon peel and sugar, then rub in the butter and knead to a smooth consistency with the egg. Place dough in a plastic bag in the refrigerator for 1 hour.

Preheat oven to 300°F. You will need a 9 or 9 1/2-inch springform pan.

Roll out the dough on a well-floured work surface, then form it into a ball again and roll it out a second time. (This dough is the worst behaved of any I have ever had to deal with, but the double rolling helps tame it, and the end result IS worth the hassle).

Place dough in bottom of a 9 or 9 1/2-inch springform pan, pressing it up the sides almost to the top. Set aside.

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.

Whisk 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 shell and gently wobble the pan back and forth until the surface is smooth. Trim the dough, leaving about 1-inch.

Bake in preheated 300°F oven for 50 to 60 minutes (longer if necessary) until well risen and golden - it resembles a souffle at this point (It will sink in the middle quite dramatically - don't worry, it's supposed to do this).

Turn the oven off, and let the cheesecake rest in the oven for 15 minutes; then remove it from the oven, cool for an hour or so at room temperature, and refrigerate for several hours before releasing sides of pan and serving.

Keep cheesecake stored in the refrigerator, covered.