In the bottom of a tall glass (about 12 ounces), combine the syrup and cold milk. Vigorously stir until smooth and fully blended—­this is your chocolate base. Place the glass in the sink, put the spoon in the glass, and uncap the seltzer. With one hand, pour the seltzer into the glass until almost full; with the other hand, “chop” rapidly at the bottom of the glass with a long metal spoon, making quick up-­and-down motions to blend everything without disturbing the foamy head. The goal is a clear separation between the chocolatey drink and the foam. Pop in a straw and serve immediately.