In a medium saucepan, whisk together the milk, custard powder (or cornstarch), and granulated sugar until the mixture is completely smooth and free of lumps before placing it on the heat. Set the saucepan over medium heat and cook, stirring constantly with a whisk or wooden spoon to prevent sticking and scorching. After 5 to 8 minutes the mixture will thicken into a smooth, pourable custard. Remove it from the heat and stir occasionally for the first 5 minutes of cooling to prevent a skin from forming on the surface. The custard should be warm but not hot when you begin assembling.

Place the finely chopped chocolate in a heatproof bowl. Heat the 200 ml of heavy cream in a small saucepan over medium heat until it just begins to simmer around the edges — do not allow it to boil. Pour the hot cream over the chopped chocolate and let the mixture sit undisturbed for 2 minutes. Then stir slowly from the center outward until the ganache is completely smooth and glossy. Add the tablespoon of butter and stir until fully incorporated and the ganache has a silky, mirror-like finish. Set aside to cool slightly while you begin assembly.

Spread a thin, even layer of the warm vanilla custard across the bottom of your 8×8 inch serving dish — just enough to anchor the first cookie layer. Arrange a single, even layer of cookies over the custard, fitting them closely together and breaking them as needed to fill any gaps. Spread another generous layer of vanilla custard evenly over the cookies, covering them completely. Add a second layer of cookies on top, again fitting them snugly together. Pour the warm chocolate ganache slowly and evenly over the second cookie layer, spreading it gently with a spatula to reach the edges.

Refrigerate the assembled dessert for approximately 30 minutes. This brief chilling period allows the ganache layer to firm slightly before the whipped cream topping is added, preventing the two layers from mixing together.

In a cold bowl, beat the 250 ml of heavy whipping cream with an electric mixer until stiff peaks form. Spread the whipped cream evenly and generously over the chilled ganache layer, smoothing it to the edges with a spatula. Decorate with chocolate shavings, grated chocolate, or crushed cookies as desired.

Return the completed dessert to the refrigerator for at least 4 hours, with overnight chilling being strongly recommended. During this time the cookies absorb moisture from the custard, transforming from crisp biscuits into a soft, tender, almost sponge-like layer that holds the dessert together beautifully. This transformation is what defines the character of the finished dish.

Slice into squares with a sharp knife and serve chilled. Each portion should reveal clean, distinct layers — pale custard, softened cookie, dark ganache, and white cream — for a presentation as beautiful as the flavor.