Cream butter and cream cheese until light and fluffy. Add sugar, egg yolks and vanilla; mix well. Gradually add flour. The dough will be VERY soft; don’t worry, it gets firmer after chilling. Cover and chill two hours or until firm. Roll out on floured surface to ¼ inch thickness. Cut into shapes; place on greased baking sheets (or use parchment paper). Bake at 350 degrees for 10-12 minutes or until set (but not browned). Cool 5 minutes. Place on wire racks to cool. Frost as desired.