As a nondrinker this is far and away my favorite pairing. Smoking a BOTL lance with it as I type.
Bitter chinotto fruit. Some kind of italian dark citrus. Saw them growing when I visited the Cinque Terre last fall.
The drink is most similar to Moxie. Bitter and sweet, very unusual profile. I can only find them at specialty soda shops, italian delis, and BevMo!
Try it sometime.