I'm a dangerous guy: I have a bottle of Fernet Branca and have no idea what to do with it. Fernet is bitter and aromatic, to the point that the wikipedia article goes out of its way to mention that "[its] strong taste can overwhelm other ingredients."
I decided I would use tequila as the base for this cocktail, and did a quick google search, which led me to the Guadalajara recipe on Cocktail Quest:
.5 Fernet Branca
.5 Agave Nectar
2 dashes Xocatle Mole Bitters
The recipe also recommends squeezing the oils from a grapefruit peel, but I didn't have one handy.
It's yummy and a good use of Fernet Branca, should you find yourself with some to spare.