MARTHA'S MOTHER'S COOKIES

Steps:

• In a food processor, combine butter, cream cheese, flour, salt, egg yolk, and zest, if using. Process until smooth. Transfer to resealable plastic bag and flatten with the palms of your hands into a 6-inch square. Seal, and chill for at least 1 hour, and up to 2 days. (Refrigerate egg white in a sealed container until ready to bake the cookies.)
• Turn the dough out onto a very lightly floured work surface. Rolling from the center outward, roll into a 12-by-8-inch rectangle. Fold in thirds, as for a business letter. Lightly dust rolling pin with flour, and then roll dough into a 16-by-8-inch rectangle. Fold again in thirds, using a dough scraper, if necessary, to release the dough from work surface. Repeat rolling and folding once more, then roll out 1/8 inch thick
• Drape dough over rolling pin and transfer to a baking sheet. Cover with plastic wrap, and refrigerate for at least 30 minutes, and up to 2 hours.
• Meanwhile, place 2 racks just above and below the center of the oven, and preheat oven to 350 degrees. Line two baking sheets with parchment or wax paper.
• Lightly beat egg white until frothy. Place about 1/4 cup sugar in a shallow bowl. Remove dough from refrigerator. Leaving it on the baking sheet, use a 1/2-inch round cookie cutter or fine rimmed glass to cut out cookies. Brush each cookie on one side with egg white. Place a few cookies at a time in sugar, egg-white side down, to coat. Transfer to prepared baking sheets, sugared-side up. Repeat with remaining cookies, leaving 1/4-inch between cookies. The dough scraps can be gathered together, and rolled out once.
• Bake until shiny and risen, rotating pans halfway through baking, 18 to 20 minutes. Remove from oven and let stand for about 2 minutes. Use a spatula to transfer cookies to wire racks to cool completely.