Sift together all purpose non bleached flour and baking powder into a medium size mixing bowl.
Add and combine: salt, vanilla sugar, lemon zest and sugar.
Add 100 g of refrigerated butter and cut into the mix using a pastry blender until it resembles a coarse meal.
Add the egg to the mixture and toss around with wooden spoon until somewhat incorporated. Knead with your hands briefly until just held together. Work as fast as you can to prevent softening the dough. Add 1 tbsp of chilled water if necessary.
Flatten the dough into a disc, wrap in a plastic wrap and refrigerate for at least 1 hour.
Butter 230 mm (9″) spring form pan and line the bottom of the pan with parchment paper.
Take the dough out of refrigerator and let sit for 5 minutes to acclimatize.
Roll the dough between two plastic wraps, into a circular shape with an approximate diameter of 355 mm (14″).
Transfer the dough to the spring form pan, plastic wrap on the top until dough is shaped to the pan.
Peel off the plastic wrap and trim the dough sticking past the spring form side edges.
Cover the pan with plastic wrap and refrigerate while making a filling.
Filling
Preheat the oven to 180ºC (350ºF) with the rack in the middle.
Separate egg yolks and egg whites into two separate medium size mixing bowls.
Using a hand held mixer whip egg yolks at low speed gradually adding approximately 1/2 of the sugar. Increase the speed to high and mix until fluffy and tripled in volume, 5 minutes.
Whisk in lemon juice, salt and lemon zest.
Add Quark to the mixture and whisk until fully integrated and smooth.
Mix approximately 1 cup of Quark mixture with the melted butter and then add it back in and incorporate to the rest of the Quark mixture.
Add cream of tartar to egg whites and mix at high speed until soft peaks are formed.
Gradually add the sugar and mix at high speed until stiff peaks.
Mix in vanilla extract.
Transfer approximately 1/4 of whipped egg whites to the Quark mixture and stir to lighten it up.
Fold gently the rest of the egg whites using a rubber spatula. Fold just long enough until fairly uniform mixture is obtained.
Assembly
Pour the filling into a dough lined spring form, level the top using the rubber spatula.
Bake for 50-60 minutes until the top is lightly browned.
Turn the oven off and crack the oven door open. Let it cool for 2 hours.
Remove cake from the oven and cool completely on a cooling rack.
Run the sharp knife around and release the spring form. Remove the bottom pan and parchment paper.
Cover the cake with plastic film and refrigerate overnight.
Serve chilled and dusted with confectioner’s sugar. Keep refrigerated and covered.