Espresso provides depth and enhances the smokiness of the chiles.
Combine espresso and chiles in a medium saucepan over medium heat. Bring to a simmer, and cook until reduced by a little more than half and chiles are soft, 5 to 7 minutes.
Add barbecue sauce, and simmer for 5 minutes. Let cool, and refrigerate. (Discard chiles if desired. Leaving chiles in the sauce will add flavor and heat. Remove chiles when desired flavor is achieved. You can also puree them, seeds optional, with the sauce in a blender for the spiciest effect.)