Make the Oreoc crust by crushing the Oreo cookies into fine crumbs, then combine them with melted unsalted butter and press into the bottom of a 9x13-inch pan. Chill for 30 minutes.

For the cheesecake layer, beat cold cream cheese until smooth, then add heavy cream and powdered sugar and mix until fluffy. Spread evenly over the chilled crust.

Whisk egg yolks and granulated sugar in a saucepan, stir in corn starch and cocoa powder, then slowly add whole milk. Heat and stir until thickened, then remove from heat and stir in chopped chocolate.

Whip cold heavy cream until soft peaks form, then gently fold into the cooled chocolate mixture and pour over the cheesecake layer.

Whip mascarpone cheese, heavy cream, and powdered sugar until fluffy, spread over the chocolate layer and refrigerate.

Cover and chill for at least 4 hours before serving. For easier slicing, freeze for 15 minutes before cutting.