Measure flour and salt into bowl. Cut in butter until flour makes a coarse mixture. Stir in egg yolk and sour cream, mixing well. Shape dough into a ball. Cover and chill in refrigerator over night.
The next day divide dough into 3 parts. On lightly floured surface, roll each into a 12-inch circle, then cut in 12 pie-shaped wedges. Blend sugar, nuts and cinnamon.
Sprinkle each wedge with sugar mixture. Starting at wide edge, roll up. Place on greased baking sheet with point tucked underneath. Bake 375° for 20 to 30 minutes. Remove from oven. Frost at once with 1 cup powdered
sugar blended with 2 tablespoons hot water and 1/4 teaspoon vanilla extract. Makes 3 dozen.