Because the dough is made with confectioners sugar, the cookies are softer and more tender than most butter cookies, but because they are rolled into logs and sliced-and-baked, they are easier to make than many of their buttery brethren.
There are just two things you must remember when you make these sablés, tips M. Lerch passed along to me. First, be gentle when you mix in the flour. Tender cookies depend on a tender touch, so you don't want to rough up the flour and activate the gluten. Second, give the logs of dough a nice long rest in the refrigerator. Refrigerating the dough relaxes the gluten and also helps the cookies hold their shape during slicing and baking.
2. Working on a smooth surface, form each piece of dough into a log that is about 1 to 1 1/4 inches (2.5 to 3.2 cm) thick. (Get the thickness right, and the length you end up with will be fine.) Wrap the logs in plastic and chill for 2 hours. (The dough can be wrapped airtight and kept refrigerated for up to 3 days or stored in the freezer for up to 1 month.)
3. Position the racks to divide the oven into thirds and preheat the oven to 350°F (180°C). Line two baking sheets with parchment paper.
4. While the oven is preheating, work on the sugar coating: Whisk the remaining egg yolk in a small bowl until it is smooth and liquid enough to use as a glaze. Spread the sugar out on a piece of wax paper. Remove the logs of dough from the refrigerator, unwrap them, and brush them lightly with a little egg yolk. Roll the logs in the sugar, pressing the sugar gently to get it to stick if necessary, then, using a sharp slender knife, slice each log into cookies about 1/4 inch (7 mm) thick. (You can make these thicker if you'd like; just bake them longer.) Place the cookies on the lined baking sheets, leaving about 1/2 inch (1.5 cm) space between them.
5. Bake the cookies for 12 to 14 minutes, or until they are set but not browned. (It's fine if the yolk-brushed edges brown a smidgen.) Transfer the cookies to cooling racks to cool to room temperature.
Keeping: Packed airtight, the cookies will keep for about 5 days at room temperature. Because the sugar coating will melt, these cookies are not suitable for freezing.
An American in Paris: I've made these tender, sweet cookies at Christmas time and dolled them up by rolling them in larger-grained sugar. They're fun rolled in crystal sugar, which gives them a bit of crunch, and pretty rolled in dazzle sugar. You could even roll them in colored sugar to add dash to a tin of assorted cookies.
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