In a large bowl, place the flour, xanthan gum, baking powder, salt and sugar and whisk to combine. Add the butter, eggs and vanilla, mixing to combine after each addition. Knead the dough with your hands until it is thick, smooth and well-integrated. Divide the dough in half, and place one half in another medium-sized bowl.

In the original bowl, add 1/4 cup flour to that half of the dough, and knead until the flour is well-incorporated into the dough. The dough will be thicker and stronger. Divide the vanilla dough in half, and place one half (now a quarter of the total dough) on a piece of unbleached parchment paper. With your hands, shape that piece of the vanilla dough into a 3-dimensional rectangle that is 1 1/2 inches tall, 1 1/2 inches wide and about 10 inches long. Square the edges as best you can. Wrap the rectangle in the parchment paper and place it in the freezer until firm (about 7 minutes). Cover the remaining piece of vanilla dough with plastic wrap and set it aside at room temperature.

To the medium-sized bowl, add the cocoa powder, and knead until the powder is well-incorporated into the dough. The dough will be a bit thicker, and will be dark brown. Divide the chocolate dough in half, and place one half (now a quarter of the total dough) on a piece of unbleached parchment paper. With your hands, shape that piece of the chocolate dough into a 3-dimensional rectangle that is 1 1/2 inches tall, 1 1/2 inches wide and about 10 inches long. Square the edges as best you can. Wrap the rectangle in the parchment paper and place it in the freezer until firm (about 7 minutes). Cover the remaining piece of chocolate dough with plastic wrap and set it aside at room temperature.

Remove the 3-dimensional rectangle of vanilla dough from the freezer and square the edges fully by pressing each flat edge against a cutting board. Unwrap the dough from the parchment paper, and, with a very sharp knife, slice the rectangle along the length into 3 equal pieces, each 1/2 inch wide. Lay each piece flat on the cutting board, and slice once again along the length into 3 equal pieces. You should have 9 strips of vanilla dough. Repeat the process with the 3-dimensional rectangle of chocolate dough. You should have 9 strips of chocolate dough. Create 2 new rectangles in a checkerboard pattern by laying 3 alternating strips of dough side-by-side, another 3 on top, and finally another 3 on top of that (see photos). Make sure each strip of dough touches the ones next to, above and below it, all along the length of the new rectangles. One checkerboard rectangle should have vanilla dough on the 4 outside corners. The other checkerboard rectangle should have chocolate dough on the 4 outside corners. Wrap each checkerboard rectangle carefully in plastic wrap and place in the freezer until firm (about 10 minutes).

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

Remove the plastic wrap from the remaining piece of chocolate dough, and place it between two pieces of unbleached parchment paper. Roll out the dough into a rectangle about 12 inches by 6 inches. Remove the top piece of parchment paper. Remove from the freezer & unwrap the checkerboard rectangle with vanilla dough on the 4 outside corners, and place it in the center of flat rectangle of rolled out chocolate dough. Wrap the chocolate dough around the checkerboard rectangle like a present, trimming it if it overlaps significantly. Square the corners. With a very sharp knife, slice the wrapped rectangle into cross-sections every 1/4 inch. Place the slices about 2 inches apart on the prepared baking sheets. Place the baking sheets in the freezer until firm (about 5 minutes).

Wrap the chocolate checkerboard rectangle & prepare the cookies.

Repeat the process with the remaining piece of vanilla dough, and the checkerboard rectangle with chocolate dough on the outside 4 corners.

One baking sheet at a time, place the cookies in the preheated oven and bake until set (and, for the vanilla-edged cookies, the edges just begin to brown), about 8 minutes. Remove the cookies from the oven and allow to cool on the baking sheet for 5 minutes, or until firm. Transfer to a wire rack to cool completely.