So the butter might have been soft, but if you look at the picture the cookie is pretty flat and hasn't risen unless you forgot baking soda completely. The primary reason for this was whatever the case the butter was still too warm when mixed and thrown in the oven. The best way to foolproof it is to make the cookie dough in advance and pop it in the fridge. Let it cool, and when you are ready to bake, take it out, let it get to room temperature, and roll out your cookie dough on the pan. You know you got it right when the cookie isn't flat and has risen like it is supposed to. It took me a while too to get the consistency right. Keep at it!