A leavening agent made of baking soda, cream of tartar and cornstarch. When combined with a liquid carbon dioxide gas bubbles are released causing a dough to rise.
There are three kinds available: double-acting (works when it gets wet and again in the oven heat), single-acting and phosphate baking powders (works in wetness). Double-acting is the most common. Check the pack date before purchase.
Keep in a cool, dry place. It is perishable, so if necessary test before using it: combine 1 teaspoon with 1/3 cup hot water. If it bubbles rapidly, it is still good.
1/2 tsp cream of tartar + 1/4 tsp baking soda = 1 tsp baking powder; 1 tsp double-acting baking powder = 1/4 tsp baking soda + 1/2 cup buttermilk, sour milk or yogurt (to replace 1/2 cup liquid in recipe); 1 tsp double-acting baking powder = 1/4 tsp baking soda + 1/4 cup molasses (reduce liquid in recipe by 1/4 cup; adjust sweeteners); 1 tsp double-acting baking powder = 2 tsp quick-acting baking powder; 1 tsp single-acting baking powder = 3/4 tsp double-acting baking powder