To begin, whisk together the melt the butter in a glass, microwave-safe bowl. Whisk together the melted butter and cocoa powder, until well combined.

In a mixing bowl fitted with the paddle attachment, cream together the butter and cocoa powder mixture with the light brown sugar and granulated sugar on medium speed until light and fluffy. Scrape down the bowl, and add eggs, one at a time, scraping down the bowl between each addition. Add the vanilla extract, and beat just until combined.

Meanwhile, in a small bowl, stir together the gluten free all-purpose flour, cassava flour, baking soda, and salt. Add the dry ingredients to the butter mixture, mixing on low until just combined. Scrape down the sides of the bowl until the flour is well incorporated.

Using a 1 ½ tablespoon-sized scoop, scoop the cookie dough onto a baking sheet, with the dough balls touching slightly. Place in the refrigerator for 1 hour, or up to 4 hours. (NOTE: The cookie dough can be made ahead of time. Simply place the dough balls in the freezer to chill for one hour. Once the dough balls are firm, place them in a zip top bag, and store in the freezer for up to 1 month.)

Place cookies in the oven, and bake for 12-15 minutes, rotating the baking sheet halfway through baking. Once the cookies are finished baking, remove from the oven, and let cool for 10 minutes before transferring to a wire cooling rack to cool completely.