Preheat your oven to 325°F (165°C). Crush 24 Oreo cookies until fine crumbs. Combine with 6 tablespoons of melted salted butter and press into a 9-inch pie plate. Bake for 12-15 minutes.

Melt 4 ounces of bittersweet chocolate with 6 tablespoons of salted butter and 1/4 cup of vegetable oil in a medium saucepan over low heat. Whisk in 1/2 cup unsweetened cocoa powder, 1 cup light brown sugar, 2 eggs, 1 teaspoon vanilla extract, and a pinch of salt. Fold in 1/2 cup all-purpose flour.

Pour the brownie batter into the cooled Oreo crust and bake for an additional 25-30 minutes. Let cool completely.

Crush 10 Oreo cookies in a bowl, mix with 1/4 cup powdered sugar, 1 tablespoon Dutch-processed cocoa powder, and a pinch of salt. Mix in 2 tablespoons of melted butter until crumbly. Spread over the brownie layer and bake for 10 minutes.

Melt 6 ounces of milk chocolate and let cool. Whip 1 cup of cold heavy cream with 2 tablespoons cocoa powder and 2 tablespoons powdered sugar until soft peaks form. Fold whipped cream into melted chocolate.

Spread the mousse layer over the brownie layer and refrigerate for at least 3 hours or overnight.

Whip an additional cup of cold heavy cream with 2 tablespoons granulated sugar until stiff peaks form. Spread over mousse layer and sprinkle with crushed Oreos.