Beat butter and sugar at medium speed until creamy. Add flour and beat at low speed until well mixed. Cover and refrigerate until firm (about 1 hour). Heat oven to 350°F. Shape dough into 1-inch balls. Place balls 2 inches apart onto ungreased cookie sheets. Make indentation in center of each cookie with thumb (edges may crack slightly). Fill each indentation with about 1/4 teaspoon jam. Bake 14-18 minutes or until edges are lightly browned. Let stand 1 minute on cookie sheets; remove to cooling rack. Cool completely.