Salt. The only thing I changed to the base of the cookie recipe was to increase the salt content. And I'm sure a little sprinkling of sea salt right before they go into the oven wouldn't hurt either.
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Refrigerate the dough before baking. The NY Times article explains more of the science behind this, but it basically just adds to the depth of flavor. I also find that it prevents the cookies from being too flat and spreading too much. I didn't leave