1 Make the pudding base: In a medium, microwave-safe bowl, whisk together the sugar, cornstarch, cocoa powder and salt until blended. Stir in 1/2 cup of the milk and whisk until smooth. Whisk in the egg yolks. Gradually stir in the remaining milk.
2 Microwave the pudding: Partially cover the bowl with plastic wrap, a paper towel, or a plate set slightly askew (so steam can escape). Microwave for 3 minutes. Remove from the microwave, take off the plastic, and whisk the pudding to distribute the heat.
Partially cover again and microwave 2 more minutes. Remove and whisk again. The pudding have started thickening.
Continue to microwave in 45-second increments, stirring the pudding between each increment. The pudding is ready when you see the mixture bubbling around the edges and it has thickened enough for the whisk to leave a light trail, or ribbon, through the pudding (it’s ok if it seems a little loose at this point; the pudding will thicken further as it cools). Total cooking time will be 6 to 8 minutes.
3 Add the chocolate, butter and vanilla: Add the chocolate, butter and vanilla to the bowl and stir with a whisk until the chocolate and butter melt and the mixture is smooth.
4 Fill the pie: Pour the filling into the pie crust. Let cool for 1 hour at room temperature. Transfer to the refrigerator and chill, uncovered, for at least 4 hours or until set. When fully set, the pudding will no longer jiggle when you nudge the pie and a paring knife inserted into the pie should emerge with just a little chocolate on it (not totally gooey).
5 When ready to serve, make the whipped cream: In the bowl of an electric mixer, stir the cream, sugar, and vanilla together. Beat on medium-high speed until the cream forms soft peaks. Spread the cream over the pie and sprinkle the top with chopped chocolate.
6 Serve: This pie is best served on the same day that it’s made, or the crust has a tendency to become soggy.