18 tablespoons (252 g) unsalted butter at cool room temperature, roughly chopped
1/2 cup (109 g) packed light brown sugar
1/2 cup (100 g) granulated sugar
1 teaspoon pure vanilla extract
3/4 teaspoon kosher salt
2 1/2 cups (350 g) all purpose gluten free flour (I used Better Batter)
1 1/4 teaspoons xanthan gum (omit if your blend already contains it)
1/2 cup + 2 tablespoons (90 g) cornstarch
6 ounces dark chocolate wafers, chopped
1 egg (any size), beaten
Coarse sugar crystals (about 1/2 cup), for rolling
In the bowl of a stand mixer fitted with the paddle attachment or a large bowl with a hand mixer, place the butter, brown sugar, granulated sugar, vanilla, and salt, and beat on medium-high speed until very light and fluffy (about 3 minutes), stopping about halfway to scrape down the bowl. The mixture will lighten in color when it’s truly fluffy. In a small bowl, place the flour, xanthan gum, and cornstarch, and whisk to combine. Add most of the flour mixture to the beaten butter and sugar mixture, reserving about 2 tablespoons in the bowl. Beat again until well-combined. The dough will seem crumbly at first, but just keep mixing until it comes together. Add the chopped chocolate to the bowl with the reserved dry ingredients and toss to coat, then transfer to the cookie dough. Mix again until the chocolate is evenly distributed throughout the cookie dough.
Divide the dough into two equal portions and place each on its own large sheet of plastic wrap. Press each piece of dough firmly together into a log about 2-inches in diameter and about 7-inches long. Wrap tightly in plastic wrap and refrigerate until mostly firm, about an hour.
When you’re ready to bake the cookies, preheat your oven to 350°F. Line rimmed baking sheets with unbleached parchment paper and set them aside. Remove one piece of the dough from the refrigerator and unwrap the plastic wrap. Brush the entire log with the beaten egg in a thin layer and sprinkle the top of the log generously with the coarse sugar crystals. Using the plastic wrap, press the crystals into the dough. Turning the dough as many times as necessary, coat the entire log in the coarse sugar crystals. Using a large serrated knife, slice the dough by cross-section into about 8 disks, each a bit more than 1/2-inch thick. If the dough seems to be crumbling at the first cut, allow the dough to sit at room temperature for about 10 minutes and try again. Place the disks about 1-inch apart on the prepared baking sheets, flat side down, and reshape them as necessary into integrated rounds. Repeat with the other log of cookie dough.
Place the baking sheets, one at a time, in the center of the preheated oven and bake for about 11 minutes or until the cookies are very lightly brown on the edges and set (not glistening) in the center. Remove from the oven and allow to cool completely on the baking sheet. When the cookies first come out of the oven, if any seem misshapen, use a butter knife or soup spoon to coax the edges back into shape. Then allow the cookies to cool until set.