Beat the butter until smooth and creamy. Add the sugar, salt, and lemon zest and mix for another minute, until thoroughly combined. Then mix in the egg yolks.
Gradually add the flour and beat until the dough looks moist. Divide the dough in half and roll each half into 8-inch log. Wrap logs in parchment paper and refrigerate overnight.
Preheat oven to 350 degrees F.
When you're ready to bake, grease a few cookie sheets or line them with parchment paper. Pull out the dough and brush the egg white over the outside. Sprinkle the sanding sugar all over the log, rolling it around a bit to make sure the sugar sticks.
Using a sharp knife, cut the log into 1/4-inch-thick slices and place them on cookie sheets about 1 inch apart.
Bake for 12 to 15 minutes, until the cookies are brown around the edges.
Reprinted with permission from The Cookiepedia, by Stacy Adimando, copyright (c) 2011. Published by Quirk Books.