In a large bowl of an electric mixer, cream the butter with the sugar, salt and egg yolk until smooth. Blend in the flour until the mixture blends into a smooth dough. Wrap the dough in a plastic bag and refrigerate 1/2 hour or up to two days.
Preheat the oven to 350 degrees. Roll out the dough on a lightly-floured surface to 1/8-inch thickness. Cut into desired shapes using cookie cutters. Place on lightly-greased cookie sheets. Sprinkle with decorating sugar. Bake for 12 to 15 minutes.