Preheat your oven to 325°F (163°C). In a food processor, crush the Oreo cookies into fine crumbs, then mix them with melted salted butter until fully combined. Press this mixture firmly into the bottom and ups sides of a pie plate. Bake for 12-15 minutes, or until set, and let it cool while you prepare the brownie layer.

Melt bittersweet chocolate, salted butter, and vegetable oil together in a heatproof bowl over simmering water, stirring gently until smooth. Remove from heat and whisk in unsweetened cocoa, light brown sugar, eggs, and vanilla. Fold in all-purpose flour until just combined. Pour this batter into the cooled crust and bake for 25-30 minutes until a toothpick comes out with slight moistness.

Crush more Oreo cookies and combine them with powdered sugar, Dutch cocoa powder, and a pinch of salt in a bowl. Drizzle in melted butter and mix until moistened. Spread this mixture over a baking sheet and bake for about 10 minutes at 350°F (177°C) until slightly crisp. Let cool before adding as a topping.

Melt milk chocolate in a microwave-safe bowl, stirring every 30 seconds until smooth. Whip cold heavy cream with cocoa powder and sugar until soft peaks form, then gently fold it into the cooled chocolate. Spread this mousse layer over the cooled brownie base.

Refrigerate the pie for at least 3 hours, preferably overnight, to allow the layers to set and flavors to harmonize.

Before serving, beat heavy cream with granulated sugar until stiff peaks form then spread it over the mousse layer. Finish with the crushed Oreo topping.