For the Crust

1. Adjust oven rack to middle position and heat oven to 325 degrees.
2. Place Oreo cookies in a food processor and process until they reach a fine powder, about 30 seconds.
3. Add melted butter and pulse until combined, about 6 pulses.
4. Using the bottom of a dry measuring cup, press the Oreo crumbs firmly into the bottom and up the sides of a 9-inch pie plate. Bake until set, about 15 minutes. Transfer pie plate to a wire rack to cool.

For the Brownie Layer

1. ­Combine chocolate, butter, oil, and cocoa in a bowl and microwave at 50 percent power, stirring often, until melted, about 1½ minutes.
2. In separate bowl, whisk sugar, eggs, vanilla, and salt until smooth.
3. Whisk in chocolate mixture until incorporated.
4. Whisk in flour until just ­combined.
5. Pour the brownie batter into the crust (the crust doesn't need to be completely cool).
6. Bake until the brownie edges begin to set and a toothpick inserted into the center comes out with thin coating of batter, about 15 minutes.
7. Transfer pie to a wire rack and let cool for 1 hour, then refrigerate until fully chilled, about 1 hour more.

For the Topping

1. While the brownie layer cools, line a rimmed baking sheet with parchment paper.
2. Crush Oreo cookies in plastic resealable gallon zipper bag, seal the bag and crush Oreos into ½-inch pieces with a rolling pin.
3. Combine sugar, cocoa, salt, and crushed Oreos in a bowl.
4. Stir in melted butter until mixture is moistened and clumps begin to form.
5. Spread crumbs in even layer on prepared sheet and bake until fragrant, about 10 minutes, shaking the sheet halfway through baking to break up crumbs.
6. Transfer the baking sheet to a wire rack and cool completely.

For the Chocolate Mousse Layer

1. Once brownie layer has fully chilled, microwave milk chocolate in large bowl at 50 percent power, stirring often, until melted, about 2 minutes.
2. Let cool until lukewarm, about 10 minutes.
3. Microwave 3 tablespoons cream in small bowl until it registers 105 to 110 degrees, about 15 seconds.
4. Whisk in cocoa until fully combined.
5. Combine cocoa-cream mixture, sugar, salt, and remaining cream in the bowl of a stand mixer.
6. Fit the stand mixer with a whisk and whip the cream mixture on medium speed until it begins to thicken, about 30 seconds.
7. Increase the mixer speed to high and whip until soft peaks form, about 60 seconds.
8. Using a whisk, fold the whipped cream mixture into the melted chocolate until no dark streaks remain.
9. Spoon the chocolate mousse into the chilled pie and spread evenly.
10. Sprinkle the mousse layer with the cooled Oreo topping and refrigerate for at least 3 hours or overnight. Serve chilled.