Butter

sticks of butter stacked upButter is by far the most popular fat used in cookies and for a good reason. It adds such a richness and delicious flavor. Also, what separates butter from shortening is that it contains only 80% of fat and 20% water. This combination butter has between fat and water makes it unique: The heat from the oven during baking turns that water into steam, which causes more gluten formation, resulting in a crisper cookie (depending on how long it bakes for). However, butter also has a lower melting point, which means that the fat melts at a lower temperature and the cookies will spread and flatten out faster during baking. Finally, butter is what gives cookies that distinct creamy flavor, and who doesn’t want that!

Side Note: Butter and Shortening can be substituted for each other in cookie recipes, however, you would need to keep a few things in mind: Since butter does contain water and less fat than shortening, you’ll need to use more of it as a substitution.

Source: Serious Eats: The science of the best chocolate chip cookies