Perfect cookies are a balance between fats and proteins, softness and structure. Too much fat and the cookies will literally run off the baking sheet in the oven; too much protein and they will be tough and chewy.
Spreading: improper mix, too much sugar, too much baking soda, too low baking temperature, dough too slack.
Crumbly: improper mix, too much sugar, too much leavening, insufficient eggs, too much shortening.
Tough: improper mix, insufficient sugar.
Resting of dough provides for complete hydration of ingredients -- if it is rolled out immediately after mixing, some parts of it will be wetter than others. Proper resting allows the moisture to distribute evenly throughout the dough. For rolling, place dough between two sheets of wax paper for best control. Roll out from center to edge, using light, even strokes.
