How to make cookies crispy? I'm looking to make very crispy thin, light, airy cookies. Kind of like dog-treats but for humans.

What makes some cookies chewy and other "snap"? What makes cookies "heavy"?
In my own cookie making experience, substituting margarine for butter yielded a crispier cookie than butter.  Sometimes too crispy.   The Perfect Cookie dismisses non butter fats for cookies although they did suggest the trick of substituting a small amount of butter for liquid oil to increase chewiness.

The margarine cookies slump and get crispy and almost toasty/ fried.  If that sounds good, try making a batch of toll house cookies with margarine and see what you think.