Put the kettle on, and warm the milk and cream together either in a saucepan or microwave.
Put the sugar and cornstarch into another saucepan and sieve in the cocoa. Add the 2 tablespoons of boiling water and whisk to a paste.
Then whisk in the egg yolks, 1 at a time, followed by the warmed milk and cream, and then the vanilla extract.
Scrape down the sides of your pan and put it on the heat, cooking and whisking for about 3 to 4 minutes until the mixture thickens - if it helps, think of consistency like mayonnaise.
Take off the heat and whisk in the finely chopped chocolate, before pouring into 4 small cups or glasses each with a yield of about 2/3 of a cup.
Cover the tops of the cups with clingfilm/clingwrap (plastic wrap) to stop them from forming a skin, and then refrigerate once they are cooler. Make sure they are not still fridge-cold when you serve them. You can add a blob of cream on top if you like.