This versatile pie can morph to fit many moods. Freeze it for an icy delight, or just refrigerate instead if you're looking for something a little softer (or you run out of time). Or leave out the candy all together for something a little less rich. You decide.
Whisk together the cocoa powder, cornstarch, 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.
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