Whisk together cocoa powder, cornstarch, sugar, and salt in a small saucepan.
Mix in heavy cream and milk.
Cook over medium-high heat while constantly stirring. Keep stirring until the cocoa powder is dissolved.
Once it's dissolved, remove from heat and add in the melted chocolate.
Return to the heat, and cook until the mixture thickens and bubbles. Be sure to stir gently at this stage.
Remove the pan from the heat once more. Add in the butter and vanilla, and stir until completely dissolved.
Strain the pudding into a bowl. Cover it with plastic wrap, and chill until it's set.