From Cook's Illustrated, November & December 2008
1 large egg
10 tablespoons (1 1/4 sticks) unsalted butter, softened
1/3 cup plus 1 tablespoon (2 3/4 ounces) granulated sugar
1/4 teaspoon salt
1 teaspoon vanilla extract
1 1/2 cups (7 1/2 ounces) unbleached all-purpose flour
1 large egg white, lightly beaten with 1 teaspoon water
4 teaspoons turbinado sugar (Beth Note: Buy a box of Sugar in the Raw packets. They're inexpensive and handy to have on hand.)
Place egg in small saucepan, cover with 1 inch water, and bring to boil over high heat. Remove pan from heat, cover, and let sit 10 minutes. Meanwhile, fill small bowl with ice water. Using slotted spoon, transfer egg to ice water and let stand 5 minutes. Crack egg and peel shell. Separate yolk from white; discard white. Press yolk through fine-mesh strainer into small bowl.
In bowl of stand mixer fitted with paddle attachment, combine butter, granulated sugar, salt, and cooked egg yolk on medium speed until light and fluffy, about 4 minutes, scraping down sides of bowl and beater with rubber spatula as needed. Turn mixer to low, add the vanilla, and mix until incorporated. Stop mixer; add flour and mix on low speed until just combined, about 30 seconds. Using rubber spatula, press dough into cohesive mass.
Divide dough in half; roll each piece into log about 6 inches long and 1 3/4 inches in diameter. Wrap each log in 12-inch square of parchment paper and twist ends to seal and firmly compact the dough into tight cylinder. Chill until firm, about 1 hour.
Adjust oven racks to upper-middle and lower-middle positions and heat oven to 350 degrees. Line 2 rimmed baking sheets with parchment paper. Using chef's knife, slice the dough into 1/4-inch-thick rounds, rotating dough so that it won't become misshapen from the weight of the knife. Place cookies 1 inch apart on baking sheets. Using pastry brush, gently brush cookies with egg white mixture and sprinkle evenly with turbinado sugar.
Bake until centers of cookies are pale golden brown with edges slightly darker than centers, about 15 minutes, rotating baking sheets front to back and top to bottom halfway through baking. Cool cookies on baking sheet 5 minutes; using thin metal spatula, transfer cookies to wire rack and cool to room temperature. For chocolate sablés, follow the above recipe but reduce the flour to 1 1/3 cups (6 2/3 ounces) and include 1/4 cup (1 ounce) Dutch-processed cocoa.
Beth Note: I used coffee extract in lieu of vanilla for the chocolate sablés to make mocha sablés.
Further Beth Note: While you can indeed brush these with egg white and sprinkle them with turbinado sugar before baking, they end up looking a little bling-y with the golden sugar on the dark cookies. I brush them with egg white for a bit of sheen but skip the sugar step. The end up looking like this: