Preparation

• Cut the vanilla beans in half lengthwise and scrape the seed pulp into a small bowl; add the granulated sugar. Using your fingers, rub them together until blended.
• In a stand mixer fitted with the paddle attachment or in a large bowl with a hand mixer, mix the butter on low speed until smooth and creamy (you don’t want it to get light and fluffy), about 1 minute; mix in the salt. Add the vanilla sugar and the confectioners’ sugar and mix until smooth, about 1 minute. Scrape down the bowl as needed. Add 1 egg yolk and mix for 1 minute. Still on low speed, mix in the flour just until blended; the dough will be soft.
• Turn the dough out onto the counter and knead it gently a few times. Divide it in half and shape each half into a 9-inch log. Wrap the logs in plastic wrap and refrigerate for at least 3 hours.
• Position oven racks in the top and bottom thirds of the oven and heat the oven to 350°F. Line two baking sheets with parchment or silicone baking liners.
• Sprinkle about 1/2 cup sanding sugar onto a piece of waxed paper. Combine the remaining egg yolk with a splash of water in a small bowl and whisk with a fork. Brush each log with the egg wash and roll it in the sanding sugar until evenly coated. Trim the ends of the logs if they’re ragged. Using a knife, cut the dough into 1/2-inch-thick rounds. Put them on the baking sheets, leaving about 2 inches between rounds.
• Bake the cookies, rotating and swapping the baking sheets’ positions halfway through, until the cookies are brown around the edges and golden on the bottom, 18 to 22 minutes. Let cool on the sheets for 5 minutes; then carefully transfer to a cooling rack and let cool completely before serving. Sablés shouldn’t be eaten warm; they need to cool so that their texture will set properly.

Make Ahead Tips

Packed in an airtight container, the cookies with keep for at least 4 days. You can also freeze the unbaked logs of dough, wrapped in plastic, for up to 2 months. Let sit at room temperature for about 10 minutes before cutting and baking; there’s no need to fully defrost them.