Place the granulated sugar in a small bowl.  Cut the vanilla beans in half lengthwise and scrape the seed pulp into the bowl.  Alternatively, add the vanilla extract or paste to the 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 on low speed until smooth, about 1 minute.  Scrape down the bowl as needed.  Add the egg yolk and mix for just 1 minute. Still on low speed or by hand, 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 or parchment paper.  Refrigerate for at least 3 hours.

Position oven rack in the center of the oven and preheat the oven to 325°F.  Line a baking sheet with parchment paper or a silicone baking liner.  

Sprinkle about 1/2 cup sanding sugar onto a piece of waxed paper.  Combine the reserved egg white 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, adding more sugar if necessary.  Trim the ends of the logs if they’re ragged.  Using a knife, cut the dough into 1/2-inch-thick rounds.  Place them on the baking sheet, leaving about 2-inches between the rounds.

Bake the cookies, rotating the baking sheet position halfway through, until the cookies are very slightly brown around the edges and golden on the bottom, 18 to 22 minutes.  Let cool on the sheet 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.

Store cookies in an airtight container for up to 1 week.