It's espresso! A shot or two of straight espresso or a few tablespoons of very dark coffee along with the liquids in your recipe have the effect of enhancing the chocolate without adding much discernible coffee flavor. Just be sure to subtract the amount of coffee or espresso being used from the overall liquids in the recipe.
If you don't want to mess with proportions in the recipe, you can also dissolve one or two tablespoons of espresso powder into the liquids to get the same effect.
We're not sure exactly why espresso has this effect on chocolate. Maybe it's because coffee and chocolate share many of the same flavor characteristics, and coffee amplifies some of the darker taste elements in the chocolate. All we know is that we picked up this trick from an old episode of Barefoot Contessa and have been using it ever since!
Is this something you do as well?
Related: Five Ways to Eat: Dark Chocolate