The versatility of this excellent drink is another strength. Learning we were out of Campari when making this now, and being reassured by the internet that other amaro can substitute, I'm happy to report that Amaro del Capo (Vecchio) makes a fine Boulevardier. With Buffalo Trace, and Fot Li vermouth (a nice Spanish vermouth).
D F
I didn't see this Negroni riff called for Luxardo/Aperol instead of Campari. It is quite excellent with Campari, though I imagine much less sweet. This worked for us with excellent quality overproof blanco tequila - worth a try if you have good tequila!
Boulevernier
Vermouth alla vaniglia, Bourbon whiskey, Amaro (e.g. Averna)
Excellent, and forgiving with some experimentation here: crushed lime ice cubes, Jack Daniel's apple whiskey, with St Germain, and Soda. Delicious.
Original (or the standard Difford's recipe) is really superb. Commenting to say that swapping ingredients seems to work well and this cocktail is forgiving with mild experimentation. Brilliant with Luxardo Aperitivo, Lazzaroni Amaro, overproof triple molasses rum, and lime juice (instead of lemon).