*you can omit the xanthan gum if it is already included in the gluten-free flour blend you are using.
Preheat the oven to 325 degrees.
Line two rimmed baking sheets with unbleached parchment paper and set them aside. In a large bowl, place the flour, xanthan gum, baking soda, salt, granulated sugar, and brown sugar, and whisk to combine well (working out any lumps in the brown sugar). Add the butter, egg, and vanilla, mixing well after each addition. The dough will be soft. Add the chocolate chips and cornstarch to the cookie dough, and mix until the chips are evenly distributed throughout.
Drop the dough by the tablespoon onto the prepared baking sheets, leaving about 2 inches from one to the next as they will spread during baking. With wet fingers, press down evenly on each tablespoon of soft dough to make a disk, smoothing to ensure a circular shape. Place the first baking sheet in the center of the preheated oven, and bake, rotating once, until lightly golden brown all over, about 7-10 minutes. Allow the cookies to cool on the baking sheet until they are firm, about 5 minutes. Transfer the cooled cookies to a wire rack, and allow them to cool completely. Repeat with the remaining baking sheets, 1 at a time.