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Preheat oven to 375°F (190°C).
Crust: Melt butter in a medium saucepan set over medium-low heat. Remove from heat; stir in crumbs. Turn into a 9-inch (23 cm) shallow glass pie plate. Press evenly up sides and on the bottom of pie plate. Bake for 8 minutes in centre of preheated oven until set.
Filling: In a medium saucepan, whisk cocoa with sugar, cornstarch and salt. Whisk in an egg as best as you can; add milk. Continue to whisk until smooth and no lumps remain. Set over medium heat. Stirring continuously, bring to a boil, about 3 to 5 minutes. The filling will thicken and bubble. Reduce heat to low; simmer gently and stir for 1 minute.
Add chocolate and butter. Stir until melted. Stir in vanilla. Turn into slightly cooled pie shell.
Refrigerate for 2 hours or until cold and set.
Topping: Beat cream with sugar and vanilla until stiff peaks form. Spoon onto chilled pie. Swirl. Grate chocolate over the top if using. Refrigerate until ready to eat.. Best eaten within a day of making.