The standard version uses Fox's u-Bet Chocolate Syrup, pretty much available only in New York City. I've brought some home from the last couple of trips I made to Manhattan, courtesy of michaelmink. Regular Hershey's is too sweet to work properly, as the result is way too sweet.
Hershey's Special Dark Chocolate, however, is a different matter. Here's what seems to work well, for me:
Pour two parts syrup in a glass. (I use a glass with regular ribs, so I can't tell you how much it actually is; I'm guessing about 3 ounces.) Add three parts milk. Mix well. Add 8 parts seltzer or sparkling water (not club soda, as that has salts in it). You'll get a nice head. Be careful as it might overflow.
Purists will, no doubt, be horrified at the sacrilege of suggesting something other than u-Bet, but the taste is much closer than most attempts with sweet syrup.