Cream the butter, cream cheese and sugar together until light and fluffy. Add the egg yolk and the vanilla. Stir in the flour. Gather the dough into a ball and chill overnight.
Preheat oven to 350 degrees F (175 degrees C).
On a lightly floured board, roll dough out to a thickness of 3/16 inch. Cut into desired shapes and place onto an ungreased baking sheet.
Bake at 350 degrees F (175 degrees C) for 12 minutes or until edges are lightly browned. Let cool. Leave the cookies plain or decorate with your favorite icing.
