Which ingredient is essential for achieving a rich flavor in buttercream?

Butter is essential for achieving a rich flavor in buttercream because it serves as the primary fat component, contributing both texture and taste. The creaminess of butter enhances the mouthfeel of the frosting and provides a rich, velvety characteristic that is a signature of high-quality buttercream. The natural flavors of butter can also deepen the overall taste profile, making it more indulgent and appealing.

While sugar does provide sweetness, and eggs might be used in some variations of buttercream, particularly French buttercream, the richness that defines traditional buttercream primarily comes from the butter itself. Milk can add fluidity and adjust texture, but it does not impart the same depth of flavor as butter. Thus, butter is the key ingredient that elevates the flavor and richness of buttercream frosting.