"Nazook is a rich Persian pastry which is not too sweet, and wonderful with tea or coffee. I developed this version while trying to reproduce a commercial product from a bakery in Salem, OR. I think mine is much better. This is definitely not for people who are watching their fat intake."
Mix 1 cup sour cream, 2 teaspoons sugar, and yeast together in a bowl. Let stand for 5 minutes. Mix in egg, vinegar, and oil.
Pour 3 cups flour into a large bowl. Cut in 1/2 cup butter, working it into the flour until mixture is the consistency of meal. Stir in sour cream mixture until dough comes together.
Turn dough out onto a floured surface; knead until smooth but still slightly tacky, about 10 minutes. Cut dough into 4 pieces and wrap each piece in plastic wrap. Refrigerate until firm, at least 2 hours and up to overnight.
Preheat oven to 350 degrees F (175 degrees C).
Mix melted butter, 2 1/2 cups sugar, and vanilla extract together in a very large bowl to make filling. Stir in 4 cups flour. Beat in egg whites.
Roll a piece of dough into an 8x12-inch rectangle on a lightly floured work surface. Crumble 1/4 of the filling mixture on top, leaving a 1/2-inch border on the long sides. Cover filling with plastic wrap; flatten out with a rolling pin. Roll dough into a tight roll, starting with one of the long sides.
Cut roll into 2-inch pieces at a slight angle. Arrange pieces on a rimmed baking sheet. Push tops down gently to flatten slightly. Repeat with remaining 3 pieces of dough and filling.
Whisk egg yolks and yogurt together to make glaze. Brush glaze over pieces of dough.
Bake in the preheated oven until golden brown, 20 to 30 minutes. Let cool before serving, about 15 minutes.
I have kept these in the refrigerator for up to 1 week, warming them slightly in the microwave before eating. I'm sure they would also freeze just fine. I have also assembled just 1 or 2 pieces of dough at a time, leaving the rest of the dough and filling in the fridge for a few days, until I was ready to bake more.
Substitute sour cream for the yogurt in the glaze if desired.