Preheat your oven to 325°F. This recipe works best with a 9-inch x 9-inch nonstick English shortbread pan. (See Recipe Notes)

Chop the butter into cubes about 1 tablespoon each. Place them, with the sugar, in the bowl of a stand mixer fitted with the paddle attachment, or a large bowl with a handheld mixer.

Add the flour, and xanthan gum, and mix until just combined. The dough will seem crumbly at first, but just keep mixing until it comes together.

Place the cookie dough in plastic wrap and press it into a disk. Allow it rest at room temperature for at least 10 minutes and up to one half hour.

Unwrap the cookie cookie dough and transfer it to the prepared baking pan. Using clean hands, press the cookie dough into an even layer in the pan.

Place a piece of parchment paper on top of the pan, and press it into an even layer using the palms of your hands and/or a bowl or bench scraper.

Place the pan in the center of the preheated oven and bake until lightly golden brown on the edges and set all the way to the center (about 18 minutes). It’s set in the center when it no longer glistens.

Remove the pan from the oven allow it to cool for about 10 minutes, or until no longer hot to the touch (but still warm). The baked shortbread is easiest to work with when it’s still warm.

Run a thin spatula very carefully around the edge of the pan to free the cookies from the pan. Place a large, flat cutting board on top of the pan.

Invert both the pan and the cutting board, together, and carefully lift the pan off the cookies.

Using a large, sharp knife, slice along the perforations in the cookies that the pan has left. . Allow the cookies to cool completely.

Dust with the optional confectioners’ sugar before serving. Store any leftover cookies in a sealed glass container at room temperature or in the freezer for longer storage.