What is the main flavoring agent in a traditional Manhattan?

The main flavoring agent in a traditional Manhattan is bitters. Bitters are concentrated herbal extracts that add complexity and depth to cocktails. In the case of the Manhattan, they balance the sweetness of the vermouth and enhance the overall flavor profile of the drink. The classic recipe typically consists of whiskey (usually rye), sweet vermouth, and a couple dashes of bitters, which is essential to achieving the cocktail's distinct taste.

Using other flavoring agents like mint, peach, or chocolate would fundamentally alter the character of a Manhattan, steering it away from its traditional roots. Mint is commonly associated with drinks like the Mojito, peach may be used in cocktails for a fruity twist but isn't traditional in a Manhattan, and chocolate flavors are more aligned with dessert-style cocktails rather than the robust, spirit-forward Manhattan. Thus, bitters are indispensable for crafting an authentic Manhattan.