1. Make the chocolate syrup: combine the sugar and water in a small saucepan. Bring to a simmer over medium heat, then whisk in the cocoa powder until smooth. Remove from the heat and stir in the vanilla. Let cool completely.
2. Spoon about 2.5 tbsp of the chocolate syrup into each glass. Add 1 1/2 cups milk and a generous pinch of salt. Stir until the syrup and salt dissolves.