COMBINE flour and sugar in large bowl. Cut in butter with pastry blender or two knives until coarse crumbs form. Add beaten egg yolk and vanilla. Mix until dough holds together.
SHAPE into small balls using half teaspoonfuls. Place 1-inch apart on cookie sheet. Make small indentation in center of each cookie. Chill 20 minutes.
HEAT oven to 350 degrees F. Fill each cookie with scant 1/4 teaspoon jam, jelly or preserves. Bake 11 to 13 minutes or until just golden around the edges.
For added sparkle, unbaked cookies may be rolled in coarse bakery sugar crystals before being placed on cookie sheet to chill.