In small saucepan whisk together cornflour, cocoa powder, sugar and salt until combined. Whisk in milk until thoroughly combined.
Bring mixture to a light boil over medium heat while whisking constantly. Once pudding has thickened to desired consistency (usually a couple of minutes) remove from heat and pour into a bowl immediately. Quickly whisk in chocolate and vanilla extract.
Serve warm or place grease proof paper on top of pudding and refrigerate until ready to serve. Pudding will thicken while in the fridge.