From
Clabber Girl:
"Baking powder is used for baking. It is made up of sodium bicarbonate (baking soda); an acid salt which reacts with moisture or heat, or both – such as tartaric acid, mono-calcium or combination of acid salts; and
cornstarch (used to keep ingredients separated)."
From
theSpruceEats:
"Mix the baking soda and cream of tartar together until well combined. This will give you one tablespoon of baking powder.
If you plan to store your baking powder, add a teaspoon of cornstarch to the mixture, and stir. This will absorb any moisture from the air, and prevent the baking powder from clumping or reacting before you need it. Store in an air-tight container between uses to
keep the baking soda fresh."