Preheat your oven to 325°F. Line rimmed baking sheets with unbleached parchment paper and set them aside.
In a large bowl, place the all purpose gluten free flour blend, xanthan gum, cornstarch, salt, baking soda and granulated sugar, and whisk to combine well.
Add the light brown sugar, and whisk once again to combine, working out any lumps in the brown sugar.
Create a well in the center of the dry ingredients, and add the butter, shortening, vanilla, egg and egg yolk, mixing to combine well after each addition.
The dough will be thick and soft. Add the chips tossed with cornstarch, and mix until evenly distributed throughout the dough.
Divide the dough into pieces of about 2 1/2 tablespoons each, roll each tightly into a ball and then place about 2 inches apart on the prepared baking sheets.
Do not flatten the balls of dough.
Place the baking sheets in the refrigerator or freezer to chill until firm (about 1 hour in the refrigerator, or 10 minutes in the freezer).
Once the dough has chilled, place in the center of the preheated oven and bake for 12 minutes, or just until the balls of dough have melted and spread and the cookies are set in the center.
They will be very lightly brown around the edges, and some may even be slightly wet toward the center. Be careful not to over bake the cookies.
Remove from the oven and allow to cool for at least 10 minutes on the baking sheet or until firm.