The Undertow from Starbucks features vanilla-flavored syrup with a little milk added. Then, two shots of espresso are poured over the back of a spoon so they float atop the milk.
The drink is meant to be drunk quickly, presenting flavorful contrasts. The bitter espresso provides a delightful counterpoint to the vanilla-infused cream.
Some baristas may not be familiar with the Undertow, but it's worth asking. You can also choose different flavored syrups for a seasonal take on the drink or re-create it at home.
Since it is an espresso-based beverage, you can swap out typically dark-roasted espresso for Starbucks' brighter blonde espresso or try it with espresso ristretto.