In a saucepan, combine cocoa and boiling water. Return to a boil and then remove from heat. Add milk, sugar and vanilla. Dissolve the rennet tablet in a tablespoon of cold water. When the milk cools to body temperature add the rennet and water. Allow to set at room temperature for about an hour, then place in the fridge to chill before serving.
Rennet is sold in most supermarkets. It is a curdling agent used in making cheese. It can also be called junket.