Chill one 12-ounce straight-sided glass in the freezer for 10 minutes.

Pour the cold milk into the glass and add the chocolate syrup without stirring.

Pour in the seltzer in a strong narrow stream so a thick white foam rises above the darker liquid.

Slide a long spoon to the bottom and stir quickly 3 or 4 times, keeping the foamy head intact.

Serve immediately before the carbonation fades.