kahlua would work too. That said, you may want to make a different sauce all together for the dish rather than substituting. The coffee liqueur is probably bringing sweetness, thickness, and a touch of naughtiness to the dish in addition to coffee flavor. If you stick with the sauce, I'd follow Jeff G's advice and at least sweeten and thicken the coffee. You wouldn't need to start with strong coffee since you're reducing it, and coffee liqueur doesn't have a super-strong coffee taste anyway. Let us know what you do and how it turns out!