In a stand mixer, or with a handheld electric mixer, beat together butter and sugar, on medium speed, for about 3 minutes, or until light and fluffy.
Add egg and vanilla. Beat until just combined.
In a separate bowl, sift together flour, xanthan gum, cocoa powder, baking soda, and salt.
Add half of the flour mixture to the butter mixture and beat on low. Once combined, add the other half of the flour mixture and beat again, until just combined.
Divide the dough in half. Form the halves into disc shapes and wrap separately in 2 pieces of plastic wrap. Refrigerate for 1 hour, or up to 1 day.
Work with one disc of dough at a time. Roll out onto a floured surface to 1/8 inch thickness.
Cut out cookies into preferred shapes and place on a parchment lined cookie sheet. Refrigerate at least 30 minutes, or until firm.
Scraps can be pressed into a disc shape again and refrigerated until firm to roll out again.
Preheat oven to 350 degrees F. Bake one pan at a time for 13 minutes.
Allow cookies to cool for 5 minutes on the pan and then remove and let cool completely on a cooling rack.
Once cool, spoon a dollop of marshmallow cream in between 2 cookies, to make a sandwich. Enjoy!
