Bourbon, Campari, and sweet vermouth — the American answer to the Negroni, and many would say better.
The Boulevardier first appeared in Erskine Gwynne's cocktail recipe book of 1927 — Gwynne was the publisher of a Paris-based literary magazine called 'The Boulevardier'. The cocktail is structurally identical to the Negroni (gin, Campari, sweet vermouth) except that bourbon replaces gin.
The swap from gin to bourbon changes the drink's character significantly. Where a Negroni is crisp and botanical, the Boulevardier is rich, caramel-sweet, and slightly warmer. The bourbon's vanilla and oak notes dovetail perfectly with the vermouth's herbal bitterness. Whether it's better than the Negroni is the kind of argument bartenders have at 2am.
Use rye whiskey instead of bourbon for a drier, spicier drink. Rye's pepper notes cut through the Campari bitterness more aggressively.
Replace bourbon with a blended Scotch whisky. More complex, slightly smoky, and closer to a Rob Roy with Campari.
Replace sweet vermouth with dry vermouth and reduce Campari to ¾ oz. Considerably drier and more spirit-forward.