Mix the rice flour and corn starch then cut in the butter or shortening to form a coarse meally texture. Blend in the remaining dry ingredients. Have your muffin pans greased and ready to fill. Pour the buttermilk over the dry ingredients and stir in quickly. The batter should be fairly soft, but will be able to hold soft peaks. It will start getting very sticky as the potato flour absorbs moisture, so quickly spoon the batter into the muffin pans to about 1/2 full. Bake at 350 degrees for 15 minutes.