In a large bowl, place the all purpose flour, xanthan gum, bread flour, cornstarch, baking soda, baking powder, salt and whisk to combine well. Add the oats, granulated sugar and light brown sugar, and whisk again to combine, working to break up any lumps in the brown sugar.

Create a well in the center of the dry ingredients and add the butter, egg, egg yolk, and vanilla, mixing to combine after each addition. The dough will be thick.

Add the chocolate disks, and mix until they are evenly distributed throughout.

Wrap the cookie dough tightly in plastic wrap and refrigerate, ideally for 24 to 72 hours.

On baking day, preheat your oven to 350°F. Line two large rimmed baking sheets with unbleached parchment paper. Divide the dough into 9 balls, each about 3 1/2 ounces (the size of generous golf balls), press them down into disks about 3/4-inch tall and place about 2 inches apart from one another on the prepared baking sheets.

Place in the preheated oven and bake until golden brown all over but still soft toward the center (about 15 minutes).

Allow to cool on the baking sheet for at least 10 minutes, or until set, before transferring to a wire rack to cool completely.