Which ingredient is commonly used in a classic Old Fashioned cocktail?

In a classic Old Fashioned cocktail, bitters is a fundamental ingredient. Bitters have a unique, aromatic quality that enhances the overall flavor profile of the cocktail. They are used to impart complexity and depth, balancing the sweetness of the sugar and the strength of the whiskey. Typically, just a few dashes of bitters are added to the cocktail, which complements the other components, such as a sugar cube, whiskey (often bourbon or rye), and citrus peel.

The other ingredients mentioned do not align with the traditional recipe for an Old Fashioned. Mint leaves are commonly associated with cocktails like the Mojito, while fresh fruit puree is more typical in modern or fruit-based cocktails. Cream soda does not fit within the classic framework of an Old Fashioned, as it would introduce sweetness and carbonation that are not characteristic of this spirit-forward drink. Thus, bitters are essential in maintaining the integrity of the classic Old Fashioned.