This is a Gluten Free Heaven recipe (you can find it here). I've modified it just slightly to make it corn-free.
3 cups brown rice flour
1 cup sorghum flour
1/2 cup millet, amaranth or teff flour
2 cups potato starch
2 cups tapioca starch
3/4 cup sugar (powdered sugar works well too)
6 tsp. baking soda
3 tbsp. baking powder
4 tsp. xanthan gum
3 tsp. salt
Sift together all ingredients. Store in an airtight container in the refrigerator or freezer.