we actually tried drinking egg creams for a little while after watching Into the Spider-Verse the first time. they have no egg in them. at least according to all the recipes we found and copied, an "egg cream" is a few tablespoons of chocolate syrup diluted with a small volume of milk or half-and-half, then topped off with soda water. if you add a dash of "acid phosphate" (dilute phosphoric acid buffered with some alkali and alkaline-earth salts), you get a related drink, the "chocolate phosphate". these are old-timey soda-fountain drinks, relics of the era before mass-produced brand-name soft drinks took over the world.
I regret drink egg creams. they are...for lack of a better way of putting it...delicious in a thoroughly disgusting way. you drink one and wonder at the same time why you're doing this to yourself. I don't think "chocolate" and "carbonation" really go together, not to my palate anyway.
~Chara