1Preheat oven to 375 degrees. In large bowl, with mixer at medium speed, beat margarine, brown sugar, egg yolk and vanilla until light and creamy. At low speed add flour and salt and mix until evenly moistened.
2With hand, press dough together to form a ball. Shape dough by rounded teaspoon into 1-in balls. Place balls, 1 in apart, on ungreased large cookie sheet. Bake cookies 5 minutes.
3Working quickly, remove cookie sheet from oven and with end of wooden spoon, make small indentation in the center of each ball. Return to oven and bake cookies 8 minutes longer or until lightly browned.
4Transfer cookies to wire rack to cool completely. Repeat with remaining balls. When cookies are cool, fill each indentation with a rounded 1/4 teaspoon jam. If not serving right away, allow jam to set, then store cooies, with waxed paper between layers, in tightly overed container up to a week.