Which ingredient is used to make the Truffle Butter for the Steak Frites?

The correct choice, which indicates that black truffle sauce is the key ingredient for making Truffle Butter for Steak Frites, highlights the rich and luxurious flavor profile that truffles contribute to dishes. Truffle Butter is essentially a mixture of softened butter and truffle, typically either fresh truffles or truffle oil, which enhances the flavor and aroma of the butter with the earthy, umami notes characteristic of truffles.

Using black truffle sauce not only lends an intense truffle essence to the butter but also complements the steak perfectly, creating a decadent finishing touch for the dish. The other ingredients listed, such as shallots, cilantro, and pesto, do not align with the traditional recipes or flavor profiles associated with Truffle Butter. Instead, they belong to different culinary contexts and are utilized for their own unique tastes, but they do not impart the distinctive truffle flavor that is essential for this butter preparation.