Step 1: Making the Oreo Crust

1. Preheat your oven to 325ºF (163ºC) and place the oven rack in the center position.
2. In a food processor, pulse 16 Oreo cookies until they become fine crumbs. Add 1 tablespoon sugar and 4 tablespoons melted butter and pulse again until everything is evenly combined.
3. Transfer the crumb mixture into a 9-inch pie plate. Use your hands to firmly press the crumbs into the bottom and up the sides of the plate.
4. Bake the crust for about 15 minutes, or until fragrant. Allow it to cool on a baker’s rack while preparing the next layer.

Step 2: Preparing the Brownie Layer

1. In a microwave-safe bowl, combine 4 ounces finely chopped bittersweet chocolate, 3 tablespoons unsalted butter, 3 tablespoons vegetable oil, and 1 1/2 tablespoons cocoa powder. Microwave at 50% power, stirring frequently, until melted and smooth, about 1 minute 30 seconds.
2. In another bowl, whisk together 2/3 cup dark brown sugar, 2 large eggs, 2 teaspoons vanilla extract, and 1/4 teaspoon salt until smooth. Gradually add the melted chocolate mixture and whisk until well combined.
3. Stir in 3 tablespoons all-purpose flour just until incorporated; do not overmix.
4. Pour the brownie batter over the Oreo crust (it’s fine if the crust isn’t completely cool). Bake in the oven for about 15 minutes, or until the edges are set and a toothpick inserted into the center comes out with a thin coating of batter.
5. Let the pie cool on a rack for about 60 minutes, then transfer to the fridge to cool completely for at least another hour.

Step 3: Preparing the Topping

1. Line a large rimmed baking sheet with parchment paper. Place 6 Oreo cookies in a resealable plastic bag and crush them into 1/2–3/4 inch bits using a rolling pin.
2. In a mixing bowl, combine the crushed cookies, 2 tablespoons powdered sugar, 1 tablespoon cocoa powder, and 1/8 teaspoon salt. Mix in 2 tablespoons melted butter until the mixture is moistened and clumps begin to form.
3. Spread the crumb mixture evenly on the prepared baking sheet. Bake for about 10 minutes, stirring halfway through. Let the topping cool completely on a wire rack and break up any large clumps.

Step 4: Preparing the Mousse Layer

1. Once the brownie layer is completely cool, melt 6 ounces milk chocolate in a microwave-safe bowl at 50% power, stirring frequently until smooth (about 1 1/2–2 minutes). Allow it to cool slightly until it reaches 90º-100ºF (32º-38ºC) using an instant-read thermometer.
2. In a small microwave-safe bowl, heat 3 tablespoons of heavy cream until it reaches 105º-110ºF (40º-43ºC), about 15 seconds. Whisk in 2 tablespoons cocoa powder until smooth.
3. In the bowl of a stand mixer, combine the cocoa mixture with 2 tablespoons powdered sugar, a pinch of salt, and the remaining cream (approximately 1 cup). Whisk on medium speed until the mixture begins to thicken, then increase speed to high and whisk until soft peaks form, about 2–3 minutes.
4. Gently fold 1/3 of the whipped cream mixture into the melted chocolate, then fold in the remaining whipped cream until evenly combined. Spread the mousse evenly over the cooled brownie layer.

Finishing the Pie

1. Garnish the pie with the cooled topping and refrigerate for at least 3–4 hours or, for best results, overnight. This will allow all the flavors to meld together for an irresistibly rich dessert.