1. Preheat oven to 350°F (176°C). Grease a 9×13 inch (23×33 cm) baking pan.
2. To make the brownie, combine the butter, sugar and vanilla extract in a medium sized bowl.
3. Add eggs and mix until well combined.
4. Combine flour, cocoa, baking powder and salt in another medium sized bowl.
5. Add the dry ingredients to the egg mixture and mix until well combined.
6. Pour the batter into the prepared pan and spread evenly.
7. Bake for 24-27 minutes, or until a toothpick comes out with a few moist crumbs. Set aside to cool.
8. To make the cheesecake filling, add the cream cheese and sugar to a large mixer bowl and mix until well combined and smooth.
9. Add the milk and vanilla extract and mix until well combined. Set aside.
10. In another bowl, add the heavy whipping cream and powdered sugar to make the whipped cream. Whisk on high speed until stiff peaks form.
11. Fold about 1/3 of the whipped cream into the cream cheese mixture until combined.
12. Add the remaining whipped cream to the cream cheese mixture and fold until combined.
13. Add the Oreo crumbs and gently fold until well combined.
14. To make the additional whipped cream for layering, add the heavy whipping cream, powdered sugar and vanilla extract to a large mixer bowl. Whisk on high speed until stiff peaks form.
15. To layer everything together, cut the brownie into small pieces. Add about half of the brownie pieces to the bottom of the trifle dish.
16. Add about half of the cheesecake filling in an even layer on top of the brownies.
17. Add about 3/4 of the chopped Oreos in an even layer on top of the cheesecake filling, then drizzle with chocolate sauce.
18. Add a little more than half of the whipped cream, reserving enough to pipe some on the top of the finished trifle.
19. Add another layer of brownies and cheesecake filling.
20. Finish off the trifle with some swirls of the remaining whipped cream, the remaining chopped Oreos and another drizzle of chocolate sauce.
21. Refrigerate trifle until ready to serve. I like to let it all set together for a few hours before serving. Trifle is best when covered for 3-4 days.