Preheat oven to 325*. Mix together the flours, starch, xanthan gum and baking powder with a whisk until well-combined. Set aside. Cream together the sugars, shortening, eggs, vanilla and salt. Add flour mixture, about 1/4 at a time, stirring with a wooden spoon. Once the flour mixture is mostly incorporated, knead the dough in the bowl, slowly punching the dough down in the middle and folding the sides of the dough into the middle. Or, put the dough on a non-stick surface (like a silicone mat or a marble slab), and knead it on there. The mixture will have the consistency of Play-Doh, but not too soft. If it is too dry, add a Tbsp of water at a time until it will gather into a ball. Resist adding water if at all possible. The dough works best if it is not very moist.
After the dough is well-mixed, several things can be done with the dough. For all recipes, greasing the pan is not necessary. Unless a crispy cookie is desired, bake (preferably on insulated pans) at 325* until the edges are just golden.
As suggested in the cookbook, roll dough into very small balls and flatten to a wafer with the bottom of a sugar-dipped glass or cookie stamp. Bake until golden brown. This will produce a light, crispy cookie.
Roll into larger balls and flatten to about 1/4″ thick. Bake until edges are just golden. This will produce a soft, chewy cookie.
Divide the dough into about 4 parts, wrap each ball in plastic wrap, and refrigerate until fairly firm. Place chilled dough on a pastry mat or on wax paper, top with plastic wrap, and pat and roll out until dough is about 1/8″ to 1/4″ thick. Use cookie cutters for your desired shape(s). The thinner the dough, the more crispy the cookie.