It is a mortal sin for a baker (at least for me!) to ran out of butter...
Butter according to Fine Cooking Magazine, is the cornerstone of all baking. You can never replace the taste and the tenderness it gives to your baked goodie.
I personally experienced that using butter properly is a key to a delictable cookie. If you noticed, most recipe calls for a "room temperature" or "softened" butter, but we shouldn't be too literal about it. So what does that mean?? The experts say, ideally butter should be about 65° to 67°F... Don't frown because you don't a thermometer... You can also test your butter on your own.
Basically, when the butter just came out of the fridge, leave it out on your counter for 30-45min. Now to test it, make sure the butter is pliable but not too soft. It's sort of like rubber when you try to bend it. You can also lightly poke it and your finger creates a nice dimple on the surface and doesn't go all the way through. Another tip, once you have your butter in the bowl and you started creaming it, it shouldn't shine like its going to sweat, if it does, pop it in the freezer for a minute and check again.
Some of you may ask why do we have to make a big fuss? Well, this will answer most questions I receive from friends: why did my cookie flattened? or why is it hard? it starts with the butter baby! (But ofcourse there's more to it, like using white sugar or brown sugar? and that's my next topic hehe)
Check this illustrations from Fine Cooking Magazine and Cooks Illustrated.





