From The Essence of Chocolate: Recipes for Baking and Cooking with Fine Chocolate by John Scharffenberger and Robert Steinberg (Hyperion, 2006).
To the surprise of everyone but native New Yorkers, egg creams contain no egg. When making an egg cream, it's best to use seltzer with a lot of fizz. Make the syrup (recipe below) and keep it on hand in the fridge.
For each egg cream, place 2 tablespoons of chocolate syrup in the bottom of a tall glass. Add 2 tablespoons half-and-half, and stir to combine. Fill the glass with seltzer.
Ice Cream Soda Variation:
Top the cream and syrup mixture with 2 scoops of your favorite ice cream. Fill with Seltzer.