Butter melts very quickly, and all at once. At one temperature, it's solid, and then you blink, and it's all melted. If the butter melts before the structural elements (starches and egg proteins) set up, you'll end up with a very flat, oddly shaped wafer of a cookie. All-butter cookies tend to spread more than cookies made with all (or part) shortening. If you don't want to go the shortening route (I try and stay away from it, although it is good for some stuff), try shaping your cookies and then putting the whole tray in the fridge or freezer for thirty minutes or so--the race between butter melting and starches and proteins setting up will be more even this way, and you'll get less spread. Oh, also, don't flatten the dough much--just lightly press down. You want the dough to be thick enough that it takes the heat a while to penetrate. Meanwhile, your starches and proteins will have a chance to start setting. And don't put your dough on hot cookie sheets. If you have to re-use a cookie sheet, run it under cold water for a few seconds to cool it off. Then dry it and bake on that. Learn about The Creaming Method for making cookies, or check out all my cookie recipes.