Chocolate Filling: In a medium-sized heatproof bowl whisk together the sugar, cornstarch, cocoa powder, and salt. Then whisk in 1/3 cup (80 ml) of the milk to make a thick paste. Add the egg yolks and whisk into the cocoa paste mixture. Set aside while you heat the milk and cream. Have ready a strainer placed over a bowl as you will need to strain the pudding after it is cooked.

Pour the remaining 1 1/3 cups (320 ml) milk with the cream into a medium-sized heavy saucepan. Bring this mixture just to a boil (the milk will start to foam up) and then remove from heat. Gradually pour the hot milk into the egg mixture, whisking constantly, until the mixture is smooth. Transfer the pudding mixture to a clean, heavy bottomed medium sized saucepan and place over medium-low heat. Cook, stirring constantly, until the mixture thickens to the consistency of mayonnaise (about 3 - 5 minutes). Remove from heat and pour through the strainer to remove any lumps that may have formed during cooking. Add the finely chopped chocolate, vanilla extract, and butter, stirring gently with a rubber spatula until the mixture is smooth. Cover with plastic wrap, to prevent a skin from forming, let cool, and then chill in the refrigerator until thick (for at least four hours, or even overnight).

Shortbread Crust: Lightly butter, or spray with a non stick vegetable spray, a 9 inch (23 cm) pie or tart pan.

In your food processor, place the flour, sugar, and salt and process to combine. Add the cold butter and pulse until the pastry starts to come together and form clumps. Transfer the pastry to your pan and, using your fingertips, evenly press the pastry onto the bottom and up the sides of the pan. Gently pierce the bottom of the crust with the tines of a fork. Cover and place the pastry crust in the freezer for 15 minutes to chill. 

Preheat oven to 425 degrees F (220 degrees C) and place rack in the center of the oven. Place the tart pan on a larger baking sheet and bake the crust until golden brown, about 13 - 15 minutes. Remove from oven and place on a wire rack to cool completely. Then fill with the chilled chocolate filling.

Topping: In a chilled large bowl, with the whisk attachment, beat the cold whipping cream and sugar until stiff peaks form. Either pipe or spread the whipped cream on top of the filling. Garnish with grated chocolate if you like. Refrigerate, if not serving immediately. Serves 8-12 people.