Provided by: Chef Dorie Greenspan
Beat butter in the bowl of a mixer fitted with the paddle attachment at medium speed until it's smooth. Add confectioners' sugar, and beat until silky. Beat in 1 of the egg yolks, following it with the salt, vanilla, and lemon zest. Reduce speed, and add flour, beating just until it disappears. (It is better to underbeat than overbeat; if streaks of flour remain, blend with a spatula.) Turn the dough out onto the counter, form it into a ball, cut in half, wrap each half in plastic, and chill until firm, about 30 minutes.
Form each piece of dough into a 1 1/4-inch-diameter log. Wrap the logs in plastic, and chill for 2 hours. (The dough can be wrapped and refrigerated for up to 3 days or frozen for up to 1 month.)
Position racks to divide the oven into thirds, and preheat to 350 degrees. Line two cookie sheets with parchment paper.
Whisk the remaining egg yolk in a small bowl until it is smooth and liquid. Spread the sugar on wax paper. Remove logs from the refrigerator, unwrap, and brush lightly with egg yolk. Roll the logs in the sugar, pressing gently so it sticks. Slice them into 1/4-inch-thick rounds. (If you make them thicker, bake them longer.) Place on cookie sheets, leaving about 1/2 inch between them, and bake for 12 to 14 minutes, or until they are set but not browned. (It's fine if the yolk-brushed edges brown a bit.) Transfer to cooling racks.
Note: Because the sugar coating will melt, these cookies are not suitable for freezing. For Christmas, roll dough in larger-grained, crystal, or colored sugar.(Published 2002)