Preheat oven to 350°. In a bowl, cream butter and sugar. Add egg yolk; mix well. Stir in the flour by hand. On a lightly floured surface, gently knead dough for 2-3 minutes or until thoroughly combined. , Roll into 1-in. balls. Place 2 in. apart on greased baking sheets. Using the end of a wooden spoon handle, make an indention in the center of each. Fill each with 1/4 teaspoon preserves. , Bake for 13-15 minutes or until edges are lightly browned. Remove to wire racks. Dust warm cookies with confectioners' sugar if desired. Cool.