In a pot over medium heat, combine the milk and cream and bring them to just below a boil.
In a large mixing bowl, whisk together the sugar, cornstarch, cocoa powder, and salt until evenly mixed.
Add the egg yolk and whisk lightly, just enough to break it up. There’s too much dry mixture to fully combine yet.
Once the milk is hot and steaming, transfer it to a heatproof measuring cup. Very slowly pour it into the mixture from step 3 while whisking constantly and vigorously.
When all of the liquid has been added, pour the entire mixture back into the pot.
Place the pot over low heat and whisk continuously until the pudding thickens and just begins to bubble.
As soon as the mixture begins to boil, whisk vigorously for 30-60 seconds.
Immediately strain the pudding through a fine-mesh sieve into a clean mixing bowl to remove any lumps or bits of cooked egg.
Whisk the chocolate and butter into the chocolate pudding until both have melted completely.
Pour the warm pudding directly on top of the baked brownie layer. Use an offset spatula or the back of a spoon to spread it evenly.
Press a piece of plastic wrap directly onto the surface to prevent a skin from forming.
Chill the pie for at least 3 hours, or until the pudding is fully set and cool to the touch.