Most recipes that call for this ingredient intend for Fernet Branca Menta, but if you have a local substitute, go ahead and try it out. You can even make your own passable facsimile by combining two parts Fernet Branca with one part mint syrup or white crème de menthe.
The base is an herbal amaro digestif like standard fernet, but it’s been mixed with mint (and sometimes sugar) to create something that has been described by hardcore connoisseurs in every way from “heavenly” to “an abomination.”
One nice detail is that the mint activates the tongue-sensors your brain uses to determine if something is cold. If you’re taking shots of whiskey on a desert camping trip, add a drop of Branca Menta and watch your friends trip out. It’s fun.