Preheat oven to 350°F (176°C). Grease and flour a 9x13 inch baking pan.

In a medium bowl, whisk together melted butter, sugar, and vanilla extract for the brownies. Beat in eggs one at a time until well combined.

In a separate bowl, whisk together flour, cocoa powder, baking powder, and salt. Gradually add the dry ingredients to the wet ingredients, mixing until just combined.

Pour the batter into the prepared pan and spread evenly. Bake for 24-27 minutes, or until a toothpick inserted into the center comes out with moist crumbs attached. Let cool completely.

For the cheesecake filling, beat softened cream cheese and granulated sugar in a large bowl until smooth. Beat in milk and vanilla extract.

In a separate bowl, beat cold heavy whipping cream and powdered sugar on high speed until stiff peaks form. Gently fold about 1/3 of the whipped cream into the cream cheese mixture, then fold in the remaining whipped cream until combined. Stir in Oreo cookie crumbs.

For the whipped cream topping, beat cold heavy whipping cream, powdered sugar, and vanilla extract on high speed until stiff peaks form.

Cut the cooled brownies into bite-sized pieces.

To assemble the trifle, layer half of the brownie pieces at the bottom of a large trifle dish. Spoon half of the Oreo cheesecake filling over the brownies. Sprinkle half of the chopped Oreos over the cheesecake layer and drizzle with half of the chocolate sauce.

Add another layer of brownie pieces, followed by the remaining Oreo cheesecake filling. Sprinkle with the remaining chopped Oreos and drizzle with the remaining chocolate sauce.

Spread the whipped cream topping evenly over the top layer. Decorate with additional Oreo crumbs or chopped Oreos if desired.

Refrigerate for at least 2-3 hours to allow the flavors to meld and the trifle to set before serving.