Combine the sugar, cocoa powder, cornstarch, and salt in a large saucepan OFF of the heat.
Whisk in the milk and heavy cream.
Place the pan on the stove top and turn it on to medium heat.
Whisk in the egg yolks.
Cook until hot, bubbly, and starting to thicken, about 8-10 minutes.
Remove the pan from the heat and stir in the vanilla extract, butter, and grated chocolate until everything is melted and well blended.
Spoon the chocolate pie filling evenly into the pie shells (you may have some extra filling).
Allow to cool slightly, then wrap each pie in plastic wrap so that the plastic wrap is touching the pie filling to prevent a "skin" from forming as they set.
Place the pies in the refrigerator to chill for at least 4 hours or until set.
Add the heavy cream and sugar to a mixing bowl and using a whisk attachment, whip until soft peaks form.
Using a spoon or piping bag and top, add a dollop of whipped cream to each mini pie.
Garnish with extra grated chocolate.
