2 oz. (1/4 c.) cream of tartar
1 oz. (2 tbsp.) baking soda
1 1/4 oz. (3 tbsp.) cornstarch

Mix as much as you need following this ratio and weighing the ingredients, or use a light hand and do not pack them down if using measuring spoons and cups. Store in tightly lidded glass jar; keeps for a month or more. (Makes about 1 cup.)