Sweet vermouth
Death & Co uses a 50/50 blend of Dolin Rouge and Punt e Mes as their house vermouth. Any quality sweet vermouth works.
A bit of history
According to legend, this cocktail was created in 1919 when Italian Count Camillo Negroni asked a Florence bartender to add gin to his Americano. Although sweet vermouth is classic for this drink, many prefer it made with dry vermouth. Substitute bourbon for the gin and you have yourself a Boulevardier.