Beat butter in a large bowl at medium speed until smooth and very creamy.  Add the sugars and salt; beat until well blended, about 1 minute.  The mixture should be smooth and and velvety, not fluffy and airy.  Reduce the mixer speed to low and beat in the egg yolks, again beating until the mixture is homogenous.

Add the flour; mix on low speed, just until the flour disappears into the dough and the dough looks uniformly moist.  (If most of the flour is incorporated but you've still got some in the bottom of the bowl, use a rubber spatula to work the rest of the flour into the dough.)  The dough will not clean the sides of the bowl, nor will it come together in a ball – and it shouldn't.  You want to work the dough as little as possible.  What you're aiming for is a soft, moist clumpy (rather than smooth) dough.  Pinch it, and it will feel a little like Play-Doh.

Center a rack in the oven and preheat the oven to 350° F.  Line two baking sheets with parchment.

Transfer dough to pastry bag fitted with a 1M-star tip.  Pipe circles, or whatever shape is desired onto prepared baking sheet, leaving 1-inch between the cookies.  Bake for 11 to 13 minutes, or until a light brown on the bottom, lightly golden around the edges and pale on the top.   Remove from the oven an let the cookies rest a minute or two before carefully lifting them with a wide metal spatula onto a rack to cool to room temperature.  Yield:  3 dozen cookies.

Procedure per Greenspan's Recipe:  Scrape the dough out onto a smooth work surface, gather it into a ball, and divide it in half.  Shape each piece into a smooth log about 9-inches long:  it's easiest wo work on a piece of plastic wrap and use the plastic to help form the log.  Wrap the logs well and refrigerate them for at least 3 hours, preferable longer.  (The dough can be kept in the refrigerator for up to 3 days or frozen for up to 2 months.)

Center a rack in the oven and preheat the oven to 350° F.  Line two baking sheets with parchment.

Remove a log of dough from the refrigerator, unwrap it and place it on a piece of parchment or waxed paper.  Whisk an egg yolk until it is smooth, and brush some of the yolk all over the sides of the dough – this is the glue – then sprinkle the entire surface of the log with decorating sugar.

Trim the ends of the roll if they're ragged, and slice the log into ¼ to ½-inch-thick cookies. Place the rounds on the baking sheets, leaving an inch of space between them.

Bake one sheet at a time for 17 to 20 minutes, rotating the baking sheet at the midway point.  When properly baked, the cookies will be light brown on the bottom, lightly golden around the edges and pale on the top; they may feel tender when you tough the tip gently, and that's fine.  Remove from the oven an let the cookies rest a minute or two before carefully lifting them with a wide metal spatula onto a rack to cool to room temperature.

Repeat with the remaining log of dough, making sure the baking sheets are cool before you bake the second batch.  Yield:  About 50 cookies.