Prepare an 11-inch x 17-inch baking pan by lining it with either parchment paper or foil.

Cut the butter into small chunks and mix together with flour. Add remaining ingredients in the order that they are written. Do not overmix. (You can use a mixer, but I prefer to do it by hand.) Cover bowl and chill dough for 2 hours.

In a bowl, thoroughly combine sour cream and farmer cheese. Add flour, yolks and sugar, mixing well (again, I prefer to do this by hand).

In the bowl of an electric mixer, beat the egg whites and vanilla sugar until stiff peaks form. Fold whites into the filling mixture, combining well. Do not overmix.

Divide dough into two equal parts. Roll one piece very thinly on a floured surface and place on bottom of prepared pan (you may have to trim some edges to create an even rectangle).

Pour filling over crust. Using a spatula, spread evenly.

Roll out second piece of dough and place carefully on top of filling.

Brush top dough with egg wash. Preheat oven to 350°F.

Bake for one hour. After an hour, check to make sure that the bottom layer of dough looks baked. Extend the baking time for another 10 or 20 minutes if necessary.

After baking, cool completely before covering.