Step 1: Make the Crust

1. Preheat your oven to 325ºF (163ºC) and position an oven rack in the center.
2. Place the 16 Oreo cookies in a food processor and pulse until they turn into fine crumbs.
3. Add 4 tablespoons of melted butter and pulse until well combined.
4. Press the mixture firmly into the bottom and up the sides of a 9-inch pie plate.
5. Bake for about 15 minutes, or until the crust is fragrant. Set aside to cool on a wire rack.

Step 2: Prepare the Brownie Layer

1. In a microwave-safe bowl, combine the 4 ounces of chopped bittersweet chocolate, 3 tablespoons of unsalted butter, vegetable oil, and 1 1/2 tablespoons of cocoa powder.
2. Microwave at 50% power, stirring frequently until melted and smooth (about 1 minute and 30 seconds).
3. In another mixing bowl, whisk together the 2/3 cup of dark brown sugar, 2 large eggs, 2 teaspoons of vanilla extract, and 1/4 teaspoon salt until smooth.
4. Add the melted chocolate mixture and whisk until evenly combined.
5. Fold in the 3 tablespoons of all-purpose flour just until incorporated.
6. Pour the brownie mixture into the prepared crust.
7. Bake for about 15 minutes, or until the edges are set and a toothpick inserted into the center comes out with a slight coating of batter.
8. Cool the pie on a wire rack for an hour, then chill in the refrigerator for at least another hour.

Step 3: Make the Topping

1. Line a large-rimmed baking sheet with parchment paper.
2. Place the 6 Oreos in a resealable plastic bag and crush them into pieces about 1/2 to 3/4 inch in size using a rolling pin.
3. In a mixing bowl, combine the crushed cookies, 2 tablespoons of powdered sugar, 1 tablespoon of cocoa powder, and 1/8 teaspoon of salt.
4. Mix in 2 tablespoons of melted butter until the mixture is moistened and begins to form clumps.
5. Spread the mixture evenly on the prepared baking sheet and bake for about 10 minutes, stirring halfway through.
6. Cool completely on a wire rack, then break up any large clumps with your hands.

Step 4: Prepare the Chocolate Mousse

1. Melt 6 ounces of milk chocolate in a large microwave-safe bowl at 50% power, stirring frequently until smooth (about 1 1/2 to 2 minutes). Let it cool until it registers 90º-100ºF (32º-38ºC).
2. In a small bowl, microwave 3 tablespoons of cream until it reaches 105º-110ºF (40º-43ºC).
3. Whisk in 2 tablespoons of cocoa powder until fully combined.
4. Combine the cocoa mixture, 2 tablespoons of powdered sugar, a pinch of salt, and the remaining heavy cream (1 cup minus 3 tablespoons) in the bowl of a stand mixer with a whisk attachment.
5. Whip on medium speed until the mixture begins to thicken, then increase to high speed and whip until soft peaks form (about 2-3 minutes).
6. Gently fold 1/3 of the whipped cream mixture into the melted chocolate, then fold in the remaining whipped cream until fully combined.
7. Spread the mousse evenly over the cooled brownie layer.

Step 5: Assemble and Chill

1. Sprinkle the baked Oreo topping over the mousse layer.
2. Refrigerate the pie for at least 3-4 hours, or preferably overnight, to set.