In a small saucepan, combine 1 pint of cream, the chocolate, and 3 tablespoons of the sugar. Cook over medium-low heat, whisking frequently, until the chocolate melts.
In a bowl, combine the cornstarch and the remaining 3 tablespoons of sugar. Add the egg yolks and whisk. Bring the chocolate mixture to a simmer (it must be bubbling gently in order for the filling to properly set). Add 2 tablespoons of the simmering chocolate mixture to the egg mixture. Whisking constantly, slowly add the egg mixture to the pan. Cook, still whisking, until thickened (it should resemble the consistency of pudding), about 2 minutes. Remove from heat. Add the vanilla and butter. Whisk until the butter melts.
Pour the filling into the crust. Place a piece of plastic wrap directly against the surface of the filling. Refrigerate for at least 3 hours and up to 48 hours. Remove the plastic wrap and sprinkle with the cocoa. With an electric mixer on medium-high, beat the remaining 1 pint of cream. Serve with the pie.