Make the Brownies

1. Heat the oven to 350°F (175°C). Butter a 9×13-inch pan or line it with parchment.
2. In a saucepan over low heat, melt the butter and remove from the heat. Stir in the granulated sugar until combined. Beat in the eggs one at a time, then add the vanilla.
3. In a separate bowl, whisk together the flour, cocoa powder, salt, and baking powder. Fold the dry mix into the wet ingredients just until blended. Gently stir in the chocolate chips.
4. Spread the batter into the prepared pan and bake 25–30 minutes — a skewer should come out with a few moist crumbs. Let the slab cool completely on a wire rack before cutting into bite-sized pieces.

Make the Cheesecake Mousse

1. In a large bowl, whip the softened cream cheese with the powdered sugar and vanilla until smooth and lump-free.
2. In a chilled bowl, beat the heavy cream to stiff peaks.
3. Fold a spoonful of whipped cream into the cream-cheese mixture to loosen it, then gently fold in the rest until the texture is light and airy. Stir in the ½ cup crushed Oreos so the filling has little cookie pockets.

Assemble the Trifle

1. Chop the cooled brownies into cubes.
2. In a trifle bowl or individual glasses, start with a layer of brownie pieces, then spoon over a generous layer of cheesecake mousse. Sprinkle a handful of crushed Oreos and a light drizzle of chocolate syrup.
3. Repeat the layers (brownie → mousse → Oreo → syrup) until your dish is filled, finishing with a hem of whipped cream, extra crumbs, and an Oreo or two on top for drama.
4. Cover and refrigerate for at least 3 hours — overnight yields the best texture and melded flavor.