Preheat the oven to 350°F and lightly grease a 9-inch pie pan.
Whisk together the melted butter and granulated sugar. Add the eggs and vanilla and mix until smooth.
Add the cocoa powder, flour, and salt. Stir just until the dry ingredients disappear.
Fold the chopped Oreos into the brownie batter and spread the mixture evenly in the prepared pie pan.
Bake for 18 to 22 minutes, until the edges are set and the center remains slightly soft. Cool completely.
Crush the Oreo cookies for the mousse, keeping some pieces slightly larger for texture.
Beat the softened cream cheese, granulated sugar, and vanilla until smooth and creamy.
In a separate bowl, whip the cold heavy cream with the powdered sugar until stiff peaks form.
Gently fold the whipped cream into the cream cheese mixture, then fold in the crushed Oreos.
Spread the Oreo mousse evenly over the completely cooled brownie base.
Cover and refrigerate for at least 4 hours, or preferably overnight.
Before serving, decorate with crushed Oreos, Oreo halves, and optional whipped cream. Slice with a sharp knife and serve chilled.
