-
Adjust two oven racks to the upper and lower position. Preheat the oven to 350ºF. Line two baking sheets with parchment paper.
-
Whisk together the gluten-free flour, cocoa powder, baking soda, and salt in a medium bowl. Set aside.
-
Combine the softened butter and brown sugar in a large bowl or the bowl of a stand mixer. Mix on medium-high speed until thick and creamy. Stop the mixer and scrape down the bottom and sides of the bowl. Turn the mixer on and mix until light, about one additional minute.
-
Turn off the mixer. Add the egg and vanilla extract. Mix on medium speed until light, about one minute.
-
Turn off the mixer again. Add the dry ingredients. Mix on medium-low speed until the dough comes together.
-
Scoop the dough into balls, about 2-tablespoons each. Form into a ball and roll into the extra sugar.
-
Place 8 balls of dough onto each of the prepared cookie sheets.
-
Bake for about 10 minutes. Rotate the pans halfway through baking (at the five minute mark.) When done, the edges of the cookies will look set and middle will be puffy.
-
Remove the pans from the oven. Let the cookies cool on the pan for about five minutes. They will deflate as they cool.
-
Transfer the cookies to a wire rack. Bake remaining dough.
-
Store the cookies in an airtight container at room temperature for up to 4 days or freeze the cooled cookies in a freezer container for up to 3 months.