Adjust oven rack to middle position. Preheat oven to 425 °F. In large bowl, whisk together white rice flour, sweet rice flour, potato starch, baking powder, sugar, salt, and baking soda. Cut butter (or shortening) into flour mixture with your fingertips or a pastry cutter until no large pieces of butter remain. Mixture should resemble a coarse meal.

Using a wooden spoon, stir in one cup buttermilk. No dry flour should cling to the bottom of the bowl. If dough is dry, add reserved buttermilk, one tablespoon at a time, until dough reaches the correct consistency.

Generously white rice flour your countertop. Turn dough out onto prepared surface and pat into a rectangle, about 7-inches across and 1-inch thick. Dip cutter into flour and cut dough into rounds. Place biscuits on parchment lined rimmed baking sheets. Gather dough scrapes and re-pat. (Gluten-free dough does not get tough.)

Bake until golden brown, about 15 minutes. Remove and transfer to wire rack to cool. Serve warm or at room temperature. Biscuits are best right after they're baked. If any leftovers remain, split and toast the next day.