For 4½ dozen small cookies, or half as many 2½inch cookies:
I find these cookies taste richer when I use half unsalted and half salted butter. If you use salted butter, be sure to omit the salt called for above.
If the butter is not soft, cut it in ½-inch-thick slices, put it into a metal mixing bowl, and heat it briefly in order to melt about an eighth of it. Let it stand a few minutes for the temperature to equalize and soften the rest of the butter. Cream the butter until it is light and fluffy, add the sugar, and beat until fluffy again. Add the egg yolk and beat until light again. If you are using chocolate, melt it over hot water and then let it cool briefly. Beat the chocolate, or whatever other flavor you are using, into the butter-egg yolk mixture. Work in the flour, and the salt if you need it, until just evenly mixed. The dough will be soft, but manageable if you handle it lightly.
Shape the cookie dough into rolls any size you like. I usually make rolls about 1¼ inches in diameter, shaping the dough by rolling it in plastic wrap. After chilling it I slice it, unwrapped, to make little one-or two-bite cookies. You can also make the rolls and flatten them to make bar-shaped cookies.
Or you can divide the dough and make half of it chocolate and the other half orange or vanilla. Pat each piece of dough flat into a ¼-inch-thick long rectangle on plastic wrap. Chill them slightly until you can move them easily but the dough can still be rolled. Lay one rectangle on top of the other, leaving about ¼ inch of the bottom one sowing on one long edge. Begin a tight roll on that edge, turning the exposed ¼ inch over the top layer to start the roll. This will give you a spiral design when you slice the dough.
When the cookie dough is rolled, chill it. Slice it straight or on the diagonal a scan ¼-inch thick and lay the cookies on a baking sheet at least 1½ inches apart. Bake in a preheated 350-degree Fahrenheit oven 10 to 12 minutes, depending on their size, or until they are golden brown on the bottom and very lightly browned on top. These are best baked shortly before serving time and kept in a tightly closed container to maintain crispness.
HTML-ified version by David Lindes; available at: http://www.daveltd.com/misc/recipes/deb/butter_cookies.html.