In a saucepan, combine milk, cream, chocolate and coffee bring it to a simmer over medium heat.
In a separate bowl, stir together sugar, cornflour and cocoa.
Once the cream mixture comes to a simmer, gradually add this to the dry ingredients and whisk well.
Transfer the entire mixture back into the saucepan and bring it to a boil, whisking continuously.
Once the mixture comes to a boil, bring it off the heat and add butter and vanilla. Whisk well.
Refrigerate for 30 minutes.