From the author, Barbara Ross: Many people make these cookies and call them by various names, including gems. This version came to me from my grandmother and I think they are particularly good.
Preheat oven to 350 degrees (F).
Mix flour and sugar. Cut in butter like a piecrust. Add egg yolk and vanilla.
Mix with fingers until dough holds together.
Place in refrigerator for 20 minutes.
Roll into balls the size of a marble and set on parchment-lined cookie sheet, 1-inch apart.
Make an indentation in the center of each ball with your thumb. Fill the indentation with any jam or jelly.
Bake at 350 degrees for 15 to 20 minutes, or until golden brown.
Cool for 5 minutes on baking sheet then move to a wire rack to cool completely.
Makes about a dozen cookies, depending on the size of the balls. I almost always double the recipe.