Ingredients:

2 tablespoons cream of tartar
1 tablespoon baking soda
1 tablespoon arrowroot or tapioca flour or cornstarch

Instructions: Combine ingredients and store tightly covered. (For double-acting baking powder, use 1/4 teaspoon of baking soda and 1/2 teaspoon of cream of tartar for each teaspoon needed.)

Notes: If you are on a rotation diet, or also avoiding corn, you could replace it with another flour like tapioca or arrowroot. We have also used rice flour. Apparently the flour is just a buffer to keep the baking soda and cream of tartar from reacting before you add a liquid.