Preheat your oven to 350°F (175°C).

In a food processor, crush 16 Oreo cookies into fine crumbs.

Mix in 1 tablespoon of sugar and 4 tablespoons of melted butter until combined.

Press the mixture evenly into the bottom of a 9-inch pie dish. Bake for about 10 minutes or until set. Set aside to cool.

In a heatproof bowl, combine 4 ounces of bittersweet chocolate and 3 tablespoons of butter. Melt over a double boiler or in 30-second intervals in the microwave, stirring until smooth.

Mix in 3 tablespoons of vegetable oil, 1 1/2 tablespoons cocoa powder, and 2/3 cup of dark brown sugar.

Once combined, whisk in 2 large eggs, 2 teaspoons of vanilla extract, and 1/4 teaspoon salt until smooth.

Gently fold in 3 tablespoons of all-purpose flour, being careful not to overmix.

Pour the brownie batter into the cooled crust and spread evenly. Bake for 25-30 minutes, or until a toothpick comes out with a few moist crumbs. Let it cool.

In a mixing bowl, combine 6 crushed Oreo cookies, 2 tablespoons powdered sugar, 1 tablespoon cocoa powder, and 1/8 teaspoon salt.

Pour in 2 tablespoons of melted butter and mix until crumbly. Set aside for later use.

In a small bowl, melt 6 ounces of milk chocolate. Allow it to cool slightly while you whip the cream.

In another bowl, whip 1 cup of heavy cream until soft peaks form. Gradually add 2 tablespoons of powdered sugar, 2 tablespoons of cocoa powder, and a pinch of salt.

Fold the melted chocolate into the whipped cream until combined.

Pour the mousse over the cooled brownie layer and smooth it out. Sprinkle the cookie crumb topping all over. Chill in the fridge for at least 4 hours or overnight.