Preheat the oven to 450°F. Sift together the rice flour, cornstarch, potato starch, xanthan gum, baking powder, baking soda, and salt; whisk in the sugar.
Using a pastry cutter, cut the shortening into the dry mixture until it resembles meal.
Create a well in the center of the dry mixture and pour in the wet mixture. Using a fork stir fold together until all of the dry ingredients at the bottom is incorporated. Do not overmix.
Take the scraps and push them all together, and if desired, pinch the top seams together to prevent them from splitting open during baking. (Do not knead the dough as this causes the dough and biscuits to turn out tough. You should be able to get about 2 more biscuits out of the first scraps. Lastly, push and pinch together the last bit of dough scraps to form one additional biscuit.
Set the baking sheet on a cooling rack to prevent the bottoms from steaming (omit this step if you prefer a softer rather than a slightly crunych bottom), and baste the tops with melted butter or margarine. Serve warm.
Try substituting 3 tablespoons apple cider vinegar for the buttermilk powder.
Replace each egg white with 2 tablespoons liquid from a can of white beans and 1 tablespoon fat (butter, margarine, shortening, bacon grease, heavy cream, or light cream) for each egg yolk to make the biscuits egg-free.