Cookies usually call for one or two type of leavening, either baking soda or baking powder, which react with liquids to form a gas, causing the cookies to rise. Baking powder and baking soda are not interchangeable.
Is alkaline, needs to react with an acidic ingredient, such as buttermilk, brown sugar, lemon juice, chocolate or molasses, to create its leavening power. Less acidic doughs brown better. Because of it’s unique feature, baking soda often is added to an acidic dough not as leavening, but to neutralize the dough’s acidity and create good browning, such as in chocolate chips cookies
Unlike baking soda, baking powder does not need an acidic ingredient to react. Baking powder contains baking soda and just the right amount of acid to produce it’s leavening power. Doughs made with baking powder spread less, bake more quickly and do not brown very well.
Adds richness, moisture and structure to cookies. Yet, too many eggs can make cookies crumbly.
Source : Betty Crockers Cookie Book