Prepare the oven and crust. Position an oven rack in the center and preheat the oven to 325°F (163°C). Process 16 Oreo cookies into fine crumbs. Add the sugar and melted butter and pulse until evenly moistened.

Bake the crust. Firmly press the cookie mixture across the bottom and up the sides of a 9-inch pie plate. Bake for about 15 minutes, until fragrant. Transfer to a wire rack.

Melt the brownie chocolate. Combine the bittersweet chocolate, butter, vegetable oil, and cocoa powder in a microwave-safe bowl. Microwave at 50% power in short intervals, stirring regularly, until smooth and melted.

Mix the brownie batter. In a separate bowl, whisk the brown sugar, eggs, vanilla, and salt until smooth. Whisk in the melted chocolate mixture. Fold in the flour just until incorporated.

Bake the brownie layer. Spread the batter over the Oreo crust. Bake for approximately 15 minutes, or until the edges are set and a toothpick inserted in the center comes out with a thin coating of moist batter. Cool on a wire rack for about 1 hour, then refrigerate for at least another hour.

Make the cookie topping. Line a rimmed baking sheet with parchment paper. Crush 6 Oreo cookies into small chunks. Combine them with powdered sugar, cocoa powder, and salt, then stir in melted butter.

Bake the topping. Spread the mixture over the prepared baking sheet. Bake at 325°F (163°C) for about 10 minutes, stirring halfway through. Cool completely and break apart any oversized clusters.

Prepare the mousse chocolate. Melt the milk chocolate at 50% microwave power, stirring frequently. Let it cool until warm, ideally around 90–100°F (32–38°C).

Prepare the cream. Warm 3 tablespoons of the heavy cream to approximately 105–110°F (40–43°C). Whisk in the cocoa powder until smooth.

Whip the mousse. Add the cocoa-cream mixture, powdered sugar, salt, and remaining cold cream to a mixing bowl. Beat until thickened, then increase the speed and whip until soft peaks form.

Finish the mousse. Gently fold about one-third of the whipped cream into the melted chocolate. Fold in the remaining whipped cream until evenly combined.

Assemble and chill. Spread the mousse over the completely cooled brownie layer and scatter the chocolate cookie topping over it. Refrigerate the Mississippi Mud Pie for at least 3–4 hours, although chilling overnight gives excellent results.