Dissolve the cornstarch (cornflour) in the water and combine with the cocoa-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 it to a simmer. Simmer, stirring often, for about 10 minutes. The cocoa is ready when it thickens and is glossy and smooth.
* The consistency of the finished product should resemble chocolate pudding that didn't quite set. If you halve this recipe, you'll get just the right amount for two large mug-fulls. This cocoa is especially fantastic when you dip churros into it.