According to Giorgio Milos, master barista for illycaffe, it's gin. He writes: "The only one true enemy of coffee's taste is gin. Its herbal properties are at odds with coffee's chocolate and other key components."
We are familiar with an Irish coffee or the after-dinner drink of Bailey's liqueur on the rocks, but a well-crafted cocktail with espresso is another matter. Of course, starting with good espresso is key. But Milos has plenty of other tips, like never using more alcohol than espresso (a one-to-one ratio is the max) and avoiding too sweet or citrusy flavors.
Milos also describes a "drinkable tiramisu." You'll have to read the article to find out...
• Coffee Mixology: A Primer, at The Atlantic.