What is a defining feature of a single shot of macchiato?

A single shot of macchiato is characterized by the addition of a small amount of hot milk, which differentiates it from a straight espresso. This "stain" or mark of hot milk is meant to soften the intensity of the espresso while still maintaining its bold flavor. The primary intention of adding the hot milk is to create a drink that balances the rich and strong espresso with just a hint of creaminess without overwhelming it.

The other options do not typically define a macchiato. For instance, it is served in a similar size cup to espresso rather than a larger one, and it generally does not have foam on top. The emphasis on using hot milk rather than cold milk is also crucial, as the macchiato aims to integrate smoothness into the drink while preserving the espresso's character. Therefore, the defining feature of a single shot of macchiato is that it is marked with a dash of hot milk.