I understand that when a steak rests after cooking, the muscle fibers relax, allowing the juices to reenter the meat as described here. I've heard that while the meat relaxes and the juices flow in, they can draw melted butter in as well. Is this true? I've tried the technique and find it tastes great, but I'm not sure if its because the butter really penetrated or because the surface is just coated with it.
I'm not sure the butter penetrates, but it tastes pretty good on the surface. ;)
I think it's pretty obvious that marbling plays a huge role in juiciness, so it stands to reason that if you bard or lard the meat it would also make a difference in juiciness (and indeed, the standard filet mignon with the bacon barding would seem to bear this out).
In the end though, it's always been my experience that once you cook the juice out of something, it's gone for good. I've always considered the idea of "resting" meat to be more about letting it cool to the point where the temperature isn't going to force out juice that would otherwise linger.
It probably depends on the cut of meat. A hangar steak or something cut on the bias with big stringy bits of meat would definitely let butter into the in-between parts.
I read a quote from Nathan Mirvold, former Microsoft CTO. He is writing a massive cookbook right now (!) and made an interesting claim in the NY Times article about it -- he said that chemically external fats can't penetrate meat as the molecules are too big; as a practical example, he said you might as well braise meat in water and store in oil rather than confiting -- it does nothing to the interior chemically.
Them's fighting words to chefs, but I haven't read any rebuttals.
All that to say, I wouldn't bet anybody that you'll find butter actually inside the steak.