The Boulevardier is an early 20th-century classic built on the Negroni formula, with bourbon (or rye) in place of gin. It first appeared in Harry McElhone’s Barflies and Cocktails book in 1927. It was credited to American expat socialite Erskine Gwynne, founder of the Boulevardier literary magazine.
This is one recipe where there’s not much wiggle room in terms of the Bitter citrus apéritif you choose. Aperol simply won’t work here; it’s too sweet. Stick with original Campari, or at least something with a similarly-bitter flavor profile like Gran Classico or Contratto.
Stir with ice and strain into a chilled glass. Garnish with a twist of orange or lemon, or a brandied cherry on a pick.