Whisk together the cocoa powder, cornflour, sugar and salt in a medium saucepan. Gradually whisk in the milk in a slow stream. Whisk in the egg yolks.
Place the mixture over medium heat and cook, whisking constantly, until it just begins to boil, 6 to 7 minutes. Reduce the heat to low and cook, whisking, until thick, about 1 minute. Remove from the heat and transfer to a large bowl. Whisk in the butter and vanilla. Cover with plastic wrap, pressing directly onto the surface of the pudding to prevent a skin from forming. Cool to room temperature, about 1 hour.
Fold the chopped candy into the pudding. Whip 3/4 cup of the cream with an electric mixer just until stiff peaks form. Fold a third of the whipped cream into the pudding to lighten it, and then fold in the other two-thirds until no streaks remain. Transfer to the pie crust and freeze until firm but not frozen solid, about 3 hours.
Whip the remaining 1/2 cup cream just until soft peaks form. Spread over the pie and garnish decoratively with sliced or chopped candy.
Cook's Note: The pie can be frozen overnight and thawed at room temperature for 10 to 15 minutes before slicing; it can also be refrigerated instead of frozen.
Use the plastic insert of the pie crust, flipped upside down, as a lid to cover the pie while freezing