Preheat your oven to 350°F. Line large rimmed baking sheets with unbleached parchment paper and set them aside.

In a large bowl, place the flour, xanthan gum, cornstarch, cocoa powder, baking powder, salt, and confectioners’ sugar, and whisk to combine well.

Create a well in the center of the dry ingredients and add the butter, egg white, and vanilla. Use the back of a large spoon to press the butter into the dry ingredients, and mix until fully combined. The dough will be thick and smooth.

Scoop the dough by the heaping tablespoon (an overfull #70 ice cream scoop is useful, but two spoons work just fine) onto the prepared baking sheet, leaving about 1 1/2-inches between pieces.

Roll each piece of dough into a round between the palms of your hands and replace on the baking sheet.

With the moistened tines of a fork, press down on the top of each piece of dough until the tines leave an impression about 1/4-inch deep.

For a sparkly appearance, dip the moistened fork in coarse sugar before pressing into each piece of cookie dough.

Place the baking sheets, one at a time, in the center of the preheated oven and bake until the cookies no longer glisten and are appear firm when pressed lightly in the center.

Remove from the oven and allow to cool completely on the baking sheet before serving.

These cookies freeze very well.