There are two ways: The first: If you prefer a super foamy topping on your espresso martini, you can also include an egg white. If you use an egg white you’ll want to do a dry shake (see above) or you can whip it separately using a whisk or an immersion blender. The second: To create the crema without an egg white, you really mus shake the shaker as hard as possible so that the natural oils in the espresso combine with air bubbles creating the crema. Give the shaker several taps and swirl the shaker after shaking to ensure that foam is there.