Which ingredient is common to both an Old Fashioned and a Grapefruit Negroni?

Bitters are an essential ingredient present in both the Old Fashioned and the Grapefruit Negroni. In an Old Fashioned, bitters are used to enhance the flavor profile, adding depth and complexity to the drink. They serve to balance the sweetness of the sugar and the richness of the bourbon. In a Grapefruit Negroni, bitters contribute to the overall flavor by complementing the other ingredients, which include gin, sweet vermouth, and grapefruit juice. The use of bitters in both cocktails helps create a harmonious blend of flavors, highlighting the importance of this ingredient in cocktail mixology.

Other ingredients like bourbon, Prosecco, and cinnamon do not share a presence in both cocktails, showcasing the unique characteristics that define each drink.