Instructions

for the crust

Adjust oven rack to middle position and heat oven to 325 degrees. Pulse cookies and sugar in food processor until coarsely ground, about 15 pulses, then process to fine, even crumbs, about 15 seconds. Drizzle melted butter over crumbs and pulse to incorporate, about 5 pulses. Transfer mixture to 9-inch pie plate. Using your hands, press crumbs very firmly into bottom and up sides of pie plate. Using lightly greased bottom of dry measuring cup, press and smooth crust into even, compact layer. Bake until fragrant and set, about 15 minutes. Transfer to wire rack.

for the brownie layer

­Combine chocolate, butter, oil, and cocoa in bowl and microwave at 50 percent power, stirring often, until melted, about 1½ minutes. In separate bowl, whisk sugar, eggs, vanilla, and salt until smooth. Whisk in chocolate mixture until incorporated. Whisk in flour until just ­combined.
Pour brownie batter into crust (crust needn’t be cool at this point). Bake pie until edges begin to set and toothpick inserted in center comes out with thin coating of batter attached, about 15 minutes. Transfer to wire rack and let cool for 1 hour, then refrigerate until fully chilled, about 1 hour longer.

for the topping

Meanwhile, line rimmed baking sheet with parchment paper. Place cookies in zipper-lock bag, press out air, and seal bag. Using rolling pin, crush cookies into ½- to ¾-inch pieces. Combine sugar, cocoa, salt, and crushed cookies in bowl. Stir in melted butter until mixture is moistened and clumps begin to form. Spread crumbs in even layer on prepared sheet and bake until fragrant, about 10 minutes, stirring halfway through baking with rubber spatula. Transfer sheet to wire rack and let cool completely. Break up any large clumps with your hands to form ½-inch pieces.

for the mousse

Once brownie layer has fully chilled, microwave chocolate in large bowl at 50 percent power, stirring often, until melted, 1½ to 2 minutes. Let cool until just barely warm and registers between 90 and 100 degrees, about 10 minutes.
Microwave 3 tablespoons cream in small bowl until it registers 105 to 110 degrees, about 15 seconds. Whisk in cocoa until homogeneous. Combine cocoa-cream mixture, sugar, salt, and remaining cream in bowl of stand mixer. Fit mixer with whisk and whip cream mixture on medium speed until beginning to thicken, about 30 seconds, scraping down bowl as needed. Increase speed to high and whip until soft peaks form, 30 to 60 seconds.
Using whisk, fold one-third of whipped cream mixture into melted chocolate to lighten. Using rubber spatula, fold in remaining whipped cream mixture until no dark streaks remain. Spoon mousse into chilled pie and spread evenly from edge to edge. Sprinkle with cooled topping and refrigerate for at least 3 hours or overnight. Serve.