When served straight-up, the Monte Carlo fits into the Scaffa family of cocktails, but it's much better and more usually served on-the-rocks, so best described...
Found this interesting note of Robb Report about some variations:
This is so simple that the whiskey is entirely up to personal taste. Use whatever rye you like and it’ll be great. For that matter, you can use bourbon (called a Kentucky Colonel), aged tequila (called a Monte Carlos), cognac, calvados, aged rum, you name it. This is one of the liqueurs great charms—as long as the spirit is aged, don’t worry, the Bénédictine will make it work.
Many thanks, Lynette. I'll add a Kentucky Colonel and Monte Carlos to Difford's Guide as soon as we've been able to photograph. In the meantime, I've added notes under variation. Do you know the names of any of the other variations based on other spirits?
This is so simple that the whiskey is entirely up to personal taste. Use whatever rye you like and it’ll be great. For that matter, you can use bourbon (called a Kentucky Colonel), aged tequila (called a Monte Carlos), cognac, calvados, aged rum, you name it. This is one of the liqueurs great charms—as long as the spirit is aged, don’t worry, the Bénédictine will make it work.