Graham Cracker Crust:  Mix together the graham cracker crumbs, sugar, and melted butter. Press onto the bottom and up the sides of a well greased 9 inch (23 cm) pie or tart pan. Cover with plastic wrap and place in the refrigerator to chill while you make the filling. 

Chocolate Filling: In a large heatproof bowl whisk together the sugar, cornstarch, cocoa powder, and salt. Then whisk in 1/2 cup (120 ml) of the milk until you have a thick paste. Add the egg yolks, one at a time, whisking to blend into the cocoa paste mixture. Set aside while you heat the milk and cream. Have a fine medium-sized strainer and bowl ready near the stove as you will need to strain the filling after it is cooked.

First, rinse a medium-sized heavy saucepan with cold water and then shake out the excess water. (This prevents the milk from scorching.) Then pour in the remaining 2 cups (480 ml) milk and the cream. Bring this mixture just to a boil and then remove from heat. (The milk will foam up to the top of pan when done, so watch carefully.) Gradually pour the milk into the egg mixture, whisking constantly, until the mixture is smooth. Transfer the filling to a clean large, heavy bottomed 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, and let cool to room temperature. Pour into the graham cracker crust, cover, and refrigerate overnight.

Topping: In a large bowl, beat the whipping cream until soft peaks form. Add the sugar and beat until stiff peaks form. Either pipe or place mounds of whipping cream on top of the filling. Garnish with grated chocolate if you like. Refrigerate, if not serving immediately.

Makes 1 - 9 inch (23 cm) pie or tart.