Baking Powder – Both Baking Powder and Baking Soda (sodium bicarbonate) create a rising effect by reacting to agents in a recipe and releasing CO2 bubbles. Baking Soda needs an outside acidic agent such as sugar, vinegar or buttermilk to react with but Baking Powder contains its own acidic agent and needs only moisture to activate it. The acidic agent in Baking Powder is Cream of Tartar (potassium bitartrate) and is a natural byproduct harvested from wine making. The problem is that while Baking Soda and Cream of Tartar each have a very long shelf life, Baking powder does not. Once combined, the two agents draw on moisture in the air to become active. You can solve this problem by making your own baking powder.... here's how.

You can make baking powder by using three common kitchen ingredients.
2 teaspoons cream of tartar (potassium bitartrate)
1 teaspoon baking soda (sodium bicarbonate)
1 teaspoon corn starch (to absorb moisture and extend shelf life)
Store in a dry container with a tight fitting lid.

Another simpler idea is to substitute any quantity of baking powder called out in a recipe, with two parts cream of tartar and one part baking soda.