Mix the cocoa and sugar together.
Dissolve the cornstarch in the water and combine with the cocoa mixture in a medium sized saucepan.
Stir this until it is a smooth paste.
Begin heating this mixture, continuously stirring it with a whisk.
Gradually pour in the milk.
Continue stirring as you bring it to a simmer.
Simmer, stirring often, for about 10 minutes.
The cocoa is ready when it thickens and is glossy and smooth.