The milk needs to be very cold so place it in the freezer to chill about 15 minutes before using.
Drizzle the chocolate syrup into a glass. Pour the milk into the glass and fill with seltzer. A siphon bottle for dispensing seltzer works best. If a siphon bottle is not available, add seltzer to milk while stirring with a long narrow spoon rapidly, slowing as the glass fills. This action to the seltzer will create the foam "head" you need for a great egg cream.
Chefs Note: The glass is a KEY part to the "ideal" Egg Cream. Utilize a SODA glass (like a Coca-Cola glass) that is narrow on the bottom and flares out on the top.
Recipe courtesy of Michael Chiarello