Preheat the oven to 350°F and line a baking sheet with parchment paper.
In a large bowl, mix melted unsalted butter and cocoa powder with a spatula until smooth.
Add in the brown sugar, granulated sugar, egg, additional egg yolk and vanilla extract. Mix until thoroughly combined.
Into the same bowl, add the gluten free flour, baking soda, xanthan gum and salt. Mix cookie dough until no clumps remain. The dough will be slightly sticky and thick.
Mix in the chocolate chips until evenly distributed.
Using slightly heaped tablespoons (about 25g), roll dough into balls and place on the prepared baking sheet. Place cookies about 2” (5cm) apart as they will spread during baking.
Bake cookies for 10 minutes until the tops start to crackle, but the centers are still soft.
Remove from the cooking sheet and cool cookies on a wire rack.
