The Times gave a little insight into the history of the Egg Cream, plus some of the seedier, corrupt power plays to be egg cream (and assorted accessories) king of New York. The article says the egg cream was born on the Lower East Side, which surprised Gothamist because for some reason, we'd always assumed it was born in Brooklyn. Perhaps it was perfected in Brooklyn.
An Egg Cream recipe from Chowhound with this warning: "Never ever add chocolate earlier in the process, because if it fraternizes with the milk before seltzer is added, you get a sticky brown head instead of a fluffy white one. Blasphemy." Get the Complete Brooklyn Egg Cream Kit (see, this is why we associate egg creams with Brooklyn, but even the official NYC Tourist site says the egg cream is from Manhattan).