Back to Cookie Experiments
This is a contentious issue within the amateur baking community. For a batch of cookies, do you melt the butter or let it soften?
Again, it depends on what kind of cookie you're after. If you melt the butter, the cookie will spread faster, making it flatter and crispier than if you just soften the butter. This is great for a flat cookie recipe, but disaster for a puffy cookie recipe.
Observe the results below. On the left: Two cookies (meant to be puffy) made with melted margarine. On the right, the exact same recipe, made with softened margarine. The melted margarine allowed the edges of the cookie to spread in the oven, producing those hideous burned rims.
Please note: the difference between these two cookies is extreme - the melted margarine one is downright ugly - but I really did try to produce pretty cookies for both of them. I didn't know beforehand what they would look like, so I put the same effort into both. I'm frankly stunned by the difference.
Conclusions: Unfortunately, since this factor depends on other parts of the cookie recipe (i.e., whether it is meant for puffy or flat cookies), you'll only know whether you should have melted or softened after you've baked. A general rule of thumb, though: melting the butter is a gamble. It could cause the edges of the cookie to spread too fast, producing that ugly burned ring. Or it could help the cookie flatten and crisp up. Experiment.
One thing that is obvious, though: if you melt the butter/margarine, make sure you let it cool slightly before mixing in the other ingredients. If the dough is warm when you mix in the chocolate chips, the chips will melt.