In a saucepan off the heat, combine the sugar, cornstarch, and flour. Add the egg yolks and whisk until smooth.
Gradually whisk in the hot milk. Bring to a boil over medium heat, whisking constantly and taking care to scrape the bottom and corners of the pan. Simmer for 1 to 2 minutes over low heat. Remove from the heat and add the chocolate and butter. Let stand for two minutes. Stir with a whisk until the pastry cream is smooth. Cover with plastic wrap directly on the hot pastry cream to prevent a skin frim forming on the surface. Let cool. Refrigerate for about 3 hours.
In a bowl, whip the cream, sugar, and vanilla with an electric mixer until stiff peaks form. Set aside.
In a 20-cm (8-inch) square serving dish, pour half the chocolate pastry cream. Layer with 9 graham crackers. Cover with half the whipped cream. Continue with the remaining ingredients. This dessert must rest overnight in the refrigerator to soften the crackers. Just before serving, dust with the cocoa powder.
Recipes with the "Healthy Pick" stamp have been evaluated by a registered dietitian member of the Ordre professionnel des diététistes du Québec.