In a small bowl, combine the cocoa powder, sugar, vanilla extract, and salt.
Pour the hot water over the dry ingredients. Stir vigorously until the cocoa and sugar are completely dissolved, forming a smooth chocolate syrup base.
Pour the chocolate syrup base into a glass.
Add the cold milk to the glass.
Stir well until the milk is uniformly colored and mixed.
Serve immediately over ice if desired.
Notes
For a less sweet drink, reduce the sugar to 1 tablespoon or substitute with a liquid sweetener like maple syrup or honey.
To make a shelf-stable chocolate milk powder mix, triple the dry ingredients (cocoa, sugar, salt) and store the mix in an airtight container. Use 3 tablespoons of the mix per 1 cup of milk.
For a creamy texture, use whole milk or add 1 teaspoon of heavy cream to the final mixture.
To make whipped chocolate milk, mix the syrup base with only 1/4 cup of milk and whip vigorously with a small whisk or milk frother until foamy before adding the remaining cold milk.