Prepare the Egg Mixture: In a medium bowl, whisk together the egg yolks and sugar until smooth and creamy. Set aside.
Mix Dry Ingredients: Sift the cocoa powder and corn starch into a separate bowl to remove any lumps, then set aside.
Heat the Milk: In a saucepan, gently heat the milk over medium heat, stirring occasionally.
Combine Wet and Dry Ingredients: Gradually add the sifted cocoa powder and corn starch mixture to the warm milk while stirring constantly until fully combined and smooth.
Temper the Eggs: Slowly pour the hot milk mixture into the egg yolk mixture, whisking constantly to avoid curdling the eggs.
Cook the Custard: Return the combined mixture to the saucepan and cook over low heat, stirring constantly, until the custard thickens to a pudding-like consistency. Once thickened, remove from heat and let it cool slightly.
Assemble the Dessert: In individual dessert cups or a large serving dish, layer the crushed cookies followed by the chocolate custard. Repeat the layers if necessary, finishing with a layer of custard.
Chill and Serve: Refrigerate the assembled dessert for at least 2 hours or until fully set. Just before serving, top with whipped cream if desired.
