Preheat oven to 350°F (180°C) and grease a round cake pan.
In a bowl, combine the flour and baking powder. Cut in the butter until the mixture resembles coarse crumbs. Add half of the granulated sugar and one egg. Mix until a smooth dough forms. Wrap and refrigerate for 30 minutes.
Roll out the dough and press it into the prepared cake pan, making sure to cover the bottom and sides. Prick the bottom with a fork to prevent it from puffing up during baking.
In a large bowl, combine the quark cheese, sour cream, remaining sugar, egg yolks, vanilla extract, and lemon zest. Mix until smooth.
In a separate bowl, whisk the egg whites until stiff peaks form. Gently fold the egg whites into the quark mixture until just combined.
Pour the quark mixture into the crust-lined cake pan and spread evenly.
Bake for 50-55 minutes, or until golden brown and the center is set (it should have a slight jiggle but not be liquid).
Let cool completely in the pan before dusting with powdered sugar and serving.
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