Why an Espresso Martini foams, and how to balance bitter, sweet and strong

The Espresso Martini works because three things pull in different directions: bitter espresso, sweet coffee liqueur and a neutral vodka, shaken hard until a pale foam forms on top.

Scale the spec to your glass, slide the sweetness, and flip to an alcohol-free version in the interactive below. Fresh hot espresso and a long, hard shake are the whole secret to the foam.