The original cocktail consisted of Gin, Old Tom is closest style that was available at the time, grapefruit juice and maraschinoliqueur.
FORBES: Boardwalk Empire Inspires Prohibition Era Themed Cocktails
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Cocktail menus across the country, for example, feature The Brooklyn, a perfect Manhattan that replaces sweet vermouth with orange-flavoured Amer Picon and bitters with maraschinoliqueur.