Sparkling Shortbread Cookies

DOUGH (OR USE HALF-BATCH OF BUTTER SHORTBREAD DOUGH)

1cup/230 grams unsalted butter, at room temperature
1cup/207 grams granulated sugar
1teaspoon kosher salt (such as Diamond Crystal)
1 to 2large egg yolks
2½cups/320 grams all-purpose flour (see Tip)
FOR FINISHING

1large egg white, beaten
½cup decorative sugar

Directions

1. Step 1
2. Make the dough: In the bowl of a stand mixer fitted with the paddle attachment, combine the butter, sugar and salt. Beat on low speed until incorporated and smooth, scraping down the sides of the bowl as needed, about 3 minutes. (Do not beat until fluffy, you don’t need to incorporate air into the dough.) Add the egg yolk and mix until just combined. Turn the mixer off and scrape down the sides of the bowl.
3. Step 2
4. Add the flour to the bowl all at once and scrape the bottom and sides of the bowl. Turn the mixer speed to low and beat until flour is fully incorporated, scraping the bowl again if needed, about 30 seconds. The dough will be in large crumbles.
5. Step 3
6. (If you’re starting with a half batch of Butter Shortbread Dough, start here.) Form the dough into two 9-inch logs. (If the dough is too crumbly to form into a log, mix in an additional egg yolk.) Wrap each log firmly in a strip of parchment paper and twist the ends shut. Refrigerate until the dough is firm enough to slice, about 30 minutes and up to 3 days.
7. Step 4
8. Heat the oven to 350 degrees. Line a baking sheet with parchment paper.
9. Step 5
10. Brush the surface of each log all over with the egg white. Sprinkle a sheet of parchment paper with the decorative sugar and roll the log in the sugar to completely coat. Slice each log into ½-inch rounds. Lay the rounds on the prepared baking sheet, spacing at least ½ inch apart.
11. Step 6
12. Bake until the cookies are golden at the bottom edges, rotating once halfway through baking, 18 to 22 minutes. Move the cookies to a wire rack to cool completely. Store at room temperature in an airtight container for up to 3 days. The baked cookies can also be stored frozen in an airtight container for up to 10 days. Thaw at room temperature before serving.
13. TIP
14. If using volume measurements, using a spoon, fluff and scoop the flour into a measuring cup, then scrape with a straight edge to level the surface. This will help prevent adding too much flour which can yield overly crumbly dough.