Start by making a chocolate pastry cream. In a large bowl, whisk together the sugar and egg yolks for 1–2 minutes, until the mixture is slightly fluffy. Add the cornstarch, cocoa powder, and vanilla. Whisk until completely smooth and lump-free.

Heat the milk in a medium saucepan over medium heat. Remove it just as it begins to simmer; do not let it boil.

Slowly pour the warm milk into the egg yolk mixture, whisking constantly as you pour.

Pour the mixture back into the saucepan. Cook over medium heat, whisking constantly and scraping the bottom and sides of the pan. Continue until the mixture thickens, usually about 1 minute after the first boil. The cream may look lumpy at first—keep whisking, and it will become smooth and glossy.

For an extra-smooth texture, strain the cream through a fine-mesh sieve into a clean bowl to remove any lumps or cooked egg bits.

Add softened butter in 2–3 stages, mixing well after each addition until fully incorporated. Then, pour the hot cream over the chopped chocolate. Let it sit 1–2 minutes to melt the chocolate, then fold gently with a rubber spatula until fully combined.

Transfer the cream to a shallow bowl or container. Cover the surface directly with plastic wrap to prevent skin from forming. Let it cool to room temperature.

Melt the butter in the microwave on low heat, then let it cool slightly while you make the crumbs.

Make the Oreo crumbs by either pulsing the cookies in a food processor or by putting them in a zip bag and beating them with a rolling pin. Make sure that the crumbs are fine and that there are no visible big chunks left.

Mix the crumbs with the warm butter and a pinch of salt until the mixture comes together and the crumbs are evenly coated in butter. If the mixture is too runny, place it into the fridge for a few minutes.

Spread the cookie mixture evenly and tightly in your pan. You can use your fingers for this step, or a flat-bottom glass or the back of a spoon.

Refrigerate while preparing the cheesecake layer.

Place the very cold cream cheese and heavy cream into a bowl along with the sifted powdered sugar and vanilla extract. Use an electric hand mixer to whip the mixture for about 2-3 minutes until fluffy.

Spread the cheesecake layer over the crust, smooth the top, then refrigerate while preparing the next layer.

Start with your cooled chocolate pastry cream at room temperature.

In a separate bowl, whip very cold heavy cream for a few minutes until it reaches soft to medium peaks. Avoid whipping until stiff peaks form.

Give the chocolate pastry cream a few gentle whisks to loosen it and make it easier to fold. Then, using a rubber spatula, gently fold the whipped cream into the chocolate pastry cream. Do this carefully, just until the mixture is smooth and evenly combined, do not overmix.

Spread the chocolate diplomat cream over the cheesecake layer, smooth the top, then refrigerate while preparing the next layer.

Place very cold heavy cream and mascarpone into a mixing bowl, along with the sifted powdered sugar. Use an electric hand mixer to whip the mixture for about 2-3 minutes until fluffy.

To finish, pipe or spread this whipped cream layer on top of the chocolate diplomat cream layer.

Refrigerate for at least four hours or overnight. For the cleanest cut, freeze it for 30 minutes before cutting. However, the dessert is best served chilled, not frozen. Decorate with chocolate shavings.

Any leftovers can be stored refrigerated in an airtight container for 2 days. This chocolate lasagna does not freeze well because of its creamy layers.