Pour the milk into a freezer-proof container and freeze until the milk begins to freeze a little, 40 minutes to 1 hour. It should not be frozen solid, but have milk ice floating on top and attached to the sides. The colder it is, the better your egg cream will come out.
Spoon the chocolate syrup into the bottom of a tall 10-ounce glass. Pour in the milk (it should only come about halfway up to leave room for the seltzer). Slide a long spoon into the glass, and then bounce the spoon up and down and stir just slightly to agitate the chocolate syrup at the bottom of the glass. Pour in the seltzer water while continuing the bounce the spoon. This method combines the syrup and seltzer and creates the frothy milk on top. The more you bounce the more bubbles you'll get. This is the old school method, and how those fountain shops got that amazing froth on top.
If it's not working for you - abandon ship and use an electric frother or a blender. It will still be delicious!