Baking powder is a two-in-one chemical leavening that combines a powdered alkali with a powdered acid. Since baking powder combines both of these, it eliminates the need for extra ingredients in your dough such as buttermilk, sour cream, or an acidic addition since no activation is needed when using baking powder. When it is added to a dough or batter, a chemical reaction takes place that produces carbon dioxide gas, inflating cookies. If you forget the baking powder you'll probably end up with some very flat, hard cookies.