The key to a frothy martini is to use freshly brewed espresso which creates that foam on top. Why not as pronounced, you can use Instant espresso in a pinch. You can also use egg whites. Egg whites will have very little effect on the cocktail's taste. They add a nice rich, silky, foam texture. So it is all about preference. If you want some foam but are concerned about raw egg, you can try Ms. Better's Bitters. It is vegan and actually works in creating foam!