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