8 tablespoons (112 g) European unsalted butter, at room temperature
1/4 cup + 2 tablespoons (75 g) granulated sugar
1 egg yolk (25 g), at room temperature
1 teaspoon pure vanilla extract
1/4 teaspoon kosher salt
1 cup (140 g) all purpose gluten free flour (I used Better Batter)
1/2 teaspoon xanthan gum (omit if your blend already contains it)
1/4 cup (36 g) cornstarch
Coarse sugar, for decorating
In the bowl of a stand mixer fitted with the paddle attachment or a large bowl with a hand mixer, place the butter, granulated sugar, egg yolk, and vanilla, and beat until well-combined, but not fluffy. Add the salt, flour, xanthan gum, and cornstarch, and mix until just combined. The cookie dough will be thick and relatively stiff. Transfer it from the mixing bowl to a large sheet of plastic wrap. Shape the cookie dough into a log about 1 1/2-inches in diameter. It will be too soft to shape properly, but do your best. Wrap the dough tightly in the plastic wrap and refrigerate until firm and easy to shape (at least 2 hours).
Once the dough is fully chilled, remove it from the refrigerator and shape it into a proper cylinder with flat sides. Preheat your oven to 350°F. Line a large rimmed baking sheet with unbleached parchment paper and set it aside.
Sprinkle 2 to 3 tablespoons of coarse sugar on a clean, flat surface. Moisten the outside of the log with wet hands, and roll it in the coarse sugar, pressing gently to help the sugar adhere to the cookie dough. Using a sharp knife, cut rounds by cross-section about 1/4-inch thick. Arrange the rounds about 1 1/2-inches apart on the prepared baking sheet. Reshape any of the cookies into a proper round in the palm of your hand as necessary. If the dough has warmed at all, place the baking sheet in the freezer or refrigerator until firm.
Place the baking sheet in the center of the preheated oven and bake until the tops are set and the cookies are lightly browned around the edges (about 10 minutes). Allow the cookies to cool on the baking sheet for 10 minutes before transferring to a wire rack to cool completely.