Heat oven to 400 degrees F. Combine sour cream and yeast in a small bowl. Set aside until slightly bubbly, about 10 minutes. Using a wooden spoon, stir in egg until smooth. Set aside. Using 2 knives or a pastry cutter, cut butter into flour until it resembles coarse meal. Stir in sour-cream mixture until dough comes together. On a lightly floured surface, roll out dough to 1/4-inch thick. Cut into 2 1/2-inch rounds. Transfer to ungreased cookie sheets, about 1 1/2 inches apart. Cover with a clean kitchen towel or plastic wrap. Let sit for 15 minutes. Make a thumbprint in the center of each cookie. Fill each thumbprint with 1 teaspoon preserves. Bake until edges are golden, 12 to 15 minutes. Transfer pans to rack to cool. Let cookies cool on pans for 5 minutes. Remove cookies to rack. While cookies are still warm, drizzle with sugar glaze.;
Combine sugar and butter. Add corn syrup, stirring to combine. Drizzle in milk a little at a time until the glaze has a runny consistency. Use immediately.
Yield: about 3/4 cup
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