In a medium bowl, whisk together the gluten free flour, cocoa powder, xanthan gum, salt, baking soda, and baking powder. Set aside.
Add the melted butter, brown sugar, and granulated sugar to the bowl of a stand mixer fitted with the paddle attachment. Mix on medium low speed until completely combined.
Add the egg and vanilla extract then mix to combine and the mixture is smooth and creamy and turns lighter in color. Then scrape down the bowl with a spatula.
Add the dry ingredients to sugar mixture and mix on low until all of the dry ingredients are incorporated. Mix in the chocolate chunks/chips.
Cover the bowl with plastic wrap and chill the dough for 2 hours.
Once the dough has chilled, preheat the oven to 350 degrees and line 2 baking pans with silicone mats or parchment paper.
Use a large cookie scoop (3 tablespoons) to portion the cookie dough into 18 cookies. Roll each portion into a ball then place on the prepared baking mats. Gently press down slightly to flatten the top.
Bake for 10-12 minutes (see note 3).
When the cookies come out of the oven, press a few extra pieces of chocolate into the tops of the cookies, if desired.
Cool the cookies on the pan on a wire rack for 10 minutes. Transfer the cookies to a wire rack and cool completely.