Mix cacao powder and sugar together.
Dissolve the cornstarch in the water and combine with the cacao-sugar 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 to a simmer.
Allow simmering, stirring often, for about 10 minutes. The drink is ready when it thickens and is glossy and smooth. A dollop of freshly whipped cream tops this drink off perfectly.
Makes 4 servings
Note: for festive occasions, add an ounce of brandy or rum.