Alton Brown has a great recipe -
here. It's quite tasty! I don't know that I would actually call it healthy, buuuuut I make it with organic sugar and cocoa, and no preservatives or thickeners, so it's probably a step up from pre-made. It keeps in the refrigerator for a couple of weeks in a squeeze bottle.
I boiled it a bit long one time and it turned into a wonderful fudge sauce for ice cream.
FYI - The corn syrup is to prevent the sauce from crystallizing when it gets cold. You could leave it out or use another type of sugar, like brown rice syrup.