vERRY eXXCELLENT cOOOKIE! I worked 10 years as a baker and the recipe we used at the bake shop was similar except this recipe didn't call for any cream of tartar, so I added it to my version. I added 1/2 tspn. What it does is make the cookie crack better when baking which gives it a better appeal in appearance as appose to that smooth top looking cookie which I don't like. It doesn't do anything for flavor so if you don't care about whether its smooth or cracked then don't bother with the suggestion.