Measure into a medium bowl, the gluten free 1 to 1 flour, tapioca starch, cornstarch, xanthan (if using), baking soda, and salt (if using). Mix together.
In a microwave safe bowl, melt the butter. Add the sugars and mix in with a rubber spatula or if you prefer you can use a hand mixer or stand mixer. Add the eggs, one at a time, mixing after each until well combined. Melt the 3 ounces of chocolate in the microwave for 15 - 25 seconds. Stir the melted chocolate and cocoa powder into the butter, sugar, and egg mixture. Add the vanilla.
Add the flour mixture and stir to combine. Stir in the chocolate chips.
Cover and chill the dough for 1 - 2 hours.
Preheat the oven to 350 degrees F. Scoop or form 1 ½ inch cookie balls and place six at a time on a baking sheet lined with parchment
Bake each tray for 10-12 minutes. Let cookies cool on the baking sheet for 2-3 minutes before carefully removing to a wire rack to finish cooling.
When completely cool, melt white or milk chocolate if you would like to add drizzle to the cookies. Drizzle on top of the cookies and let cool completely before storing in an airtight container between parchment. Best if eaten within 3 days.