Step 1
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.
Step 2
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.
Step 3
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.
Step 4
Preheat oven to 350 degrees F (175 degrees C).
Step 5
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.
Step 6
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.
Step 7
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.
Step 8
Whisk egg yolks and yogurt together to make glaze. Brush glaze over pieces of dough.
Step 9
Bake in the preheated oven until golden brown 20 to 30 minutes. Let cool before serving about 15 minutes.