In a medium bowl, whisk together the milk and yolks. In a heavy medium saucepan, whisk together the sugar, cocoa, cornstarch, and salt. Whisk in the milk mixture until thoroughly combined, then turn on the heat to medium. Cook, whisking constantly, until the pudding thickens and comes to a boil, about 5 minutes. Continue boiling for 1 more minute, whisking constantly so it stays smooth. Remove the pudding from the heat and add the vanilla. Cool for 10 minutes. Pour the pudding into the prepared crust. Allow the pie to cool, then cover with foil or plastic wrap and chill for at least 6 hours (and up to 24). Serve topped with freshly whipped cream.