Pulse the Oreo cookies in a food processor until they become fine crumbs, then combine them with the melted butter and press the mixture firmly across a 9 x 13-inch pan or glass dish. Pack the corners and surface evenly; the crust should resist lightly when pressed after chilling in the freezer.

Beat the softened cream cheese, milk, and creamy peanut butter on medium speed until fluffy, then gradually mix in the powdered sugar. Stop when the filling is silky and clump-free rather than grainy or streaked.

Fold in the whipped topping with a gentle hand, then spread the mousse evenly over the cold Oreo crust. Keep the spatula level to avoid pulling crumbs upward, then return the pan to the freezer while you make the pudding.

Whisk both boxes of instant chocolate pudding mix with the 2 ¾ cups milk in a bowl until fully combined. After about 2 minutes of brisk whisking, the pudding should noticeably thicken; the reduced milk amount makes it firmer than package pudding.

Spoon the pudding over the peanut butter layer and smooth it gently, then refrigerate the pan for about 5 minutes. The surface should lose its wet sheen and hold a soft, even layer before topping.

Finish with the remaining whipped topping, chopped Mini Reese's Peanut Butter Cups, peanut butter chips, and chocolate syrup optional. Refrigerate for at least 3 hours so the mousse and pudding settle into squares that hold together.