PREPARE THE COOKIE DOUGH

1. In a medium bowl, whisk together the gluten-free flour, cornstarch, salt, baking powder, and baking soda. Set aside.
2. In a large mixing bowl with an electric hand mixer or in the bowl of a stand mixer fitted with the paddle attachment, beat the softened butter, dark brown sugar, and granulated sugar on medium speed until light and creamy, about 2-3 minutes. Scrape down the sides and bottom of the bowl as needed.
3. Add the egg, egg yolk, and vanilla and mix until well combined, scraping down the bowl as needed.
4. With the mixer on low speed, gradually add the dry ingredients and mix just until combined.
5. Scoop the dough into 2 1/2 tablespoon (45-50g) portions and place on a baking sheet or plate. Cover and refrigerate for 1 hour.

BAKE THE COOKIES

1. Preheat the oven to 350°F. Line a large baking sheet with parchment paper.
2. Roll the chilled cookie dough balls in granulated sugar and arrange them on the prepared baking sheet (6 per sheet).
3. Bake for 9-12 minutes, or until the edges are set and the centers still look slightly soft.
4. Immediately after removing the cookies from the oven, gently swirl the edges with a large roung cookie cutter or a bowl that is slightly larger than the cookies.
5. Allow the cookies to cool on the baking sheet for about 5-10 minutes before transferring them to wire rack to cool completely.

Store the cookies in an airtight container at room temperature for up to 4–5 days.