In small bowl, whisk together white rice flour, potato starch, cocoa, ground chipotle, baking powder, salt, and xanthan gum. In bowl of stand mixer fitted with paddle attachment, cream together butter and granulated sugar on medium speed until thick paste forms, about 30 seconds. Stop mixer. Scrape down bottom and sides of bowl. Add egg. Cream on medium speed until light and thoroughly combined, about 30 seconds. Stop mixer. Add dry ingredients. Mix on medium-low speed until dough forms, about 2 minutes. Dough will be dry. If after two minutes, dough does not form, add one teaspoon milk.