We can thank Prohibition for this cocktail. The exiled Harry McHelone helmed bars in London before opening Harry's New York Bar in Paris. The favorite tipple of titans of French Industry and Literature, the Boulevardier was named for the French idiom meaning a "man about town." The modern fascination of Old World amari and bitters in today's market can be traced back to this period when American bartenders were exposed to liquors and cordials previously unavailable to them. This cocktail at first reminds one of the bracing brightness of a Negroni, and, as the cocktail warms, it drifts into a slightly bittered, citrusy Manhattan. Use a robust bourbon to stand up to such strong flavors.