Leavening, whether tiny bubbles of air or of carbon dioxide, helps tenderize cookies, and encourages them to puff. Many cookies are leavened only with air bubbles incorporated when the sugar is creamed with the fat, or beaten with the eggs. Some are supplemented with chemical leavenings. Alkaline baking soda may be used when the dough includes such acid ingredients as honey, brown sugar, and cake flour.
From the book On Food and Cooking (2nd edition) by Harold McGee. © 2004 Harold McGee.
By permission of Scribner, a division of Simon & Schuster, Inc.