In the bowl of an electric mixer fitted with the paddle attachment, beat the butter on medium speed until smooth and very creamy. You don’t want the mixture light and fluffy.

Combine the granulated sugar and lemon zest in a small bowl. Use your fingers to rub the mixture together until the sugar is moist and very aromatic. Add both sugars to the bowl with the butter and continue beating for 1 minute, until smooth. Reduce speed to low and beat in the 2 egg yolks.

Turn off the mixer and add all of the flour. Drape a kitchen towel over the bowl and pulse the mixer at low speed 5 times, about 2 seconds each time. If there’s still a significant amount of flour on top, then pulse a few more times. If not, remove the towel and mix for about 3o seconds, just until the flour disappears into the dough. You’re looking for a soft, clumpy dough, so don’t expect it to come together in a ball like many cookie doughs do. If it feels a bit like Play-Doh, you’re on the right track.

Scrape the dough onto a smooth work surface and gather into a ball. Divide in half, then place each piece onto a sheet of plastic wrap. Shape each piece into a smooth log about 9 inches long, using the plastic wrap to help form the log. Wrap well and refrigerate for at least 3 hours, preferably longer. The dough can be kept refrigerated for 3 days or frozen up to 2 months.

(To help keep its round shape, you can store the dough in a spent paper towel roll if you’ve got one. Simply cut a slit down the length of the tube and place your dough inside.)

When ready to bake, heat oven to 350º. Line two baking sheets with parchment or silicone baking mats.

Remove one log from the refrigerator. Whisk the egg yolk in a small bowl, then brush all over the sides of the dough. Sprinkle the entire log with sanding sugar. Use a sharp knife to slice the log into ⅓-inch thick cookies. Turing the dough a quarter turn after each slice will prevent the bottom from becoming flat. Place the rounds on the prepared baking sheets, leaving about an inch of space between them.

Bake one sheet at a time for 17-20 minutes, rotating halfway through. The cookies will be light brown on the bottom and light golden on the sides, but the tops will remain pale. It’s okay if the tops feel tender when you remove them from the oven. Allow to rest for a couple of minutes on the baking sheet, then transfer to a cooling rack to cool completely.

Repeat with remaining dough, making sure the baking sheets are cool before baking another batch.