Preheat oven to 350 degrees F. In a large bowl, cream together shortening and sugar. Add egg and vanilla extract. Mix well. Add potato starch, cornstarch, baking soda, baking powder, xanthan gum, salt and water. Mix well to eliminate any lumps in the dough. Dough will be sticky. Lightly oil hands or spray them with nonstick spray to better handle dough. Shape into small balls, using a slightly-rounded teaspoon of dough for each cookie. Roll in sugar/cinnamon mixture.
Place on lightly greased baking sheet and press each ball to an even thickness of about 1/3 inch. Bake for 10-12 minutes. A single cookie should be removed at the shortest-baking time to test for doneness. Browning is not a good indicator for these cookies.
Source: The Gluten-Free Baker by Robyn Ryberg
Recipe by The Baking Beauties at http://www.thebakingbeauties.com/2008/12/gluten-free-snickerdoodles.html