Preheat your oven to 325°F (163°C). In a food processor, pulse 16 Oreo cookies into fine crumbs. In a bowl, combine the crumbs with 4 tablespoons of melted butter and 1 tablespoon of sugar, mixing until evenly coated. Press this mixture firmly into a 9-inch pie plate, ensuring an even layer, and bake for 15 minutes. Allow the crust to cool on a wire rack before proceeding.

In a microwave-safe bowl, combine 4 ounces of chopped bittersweet chocolate, 4 tablespoons of butter, 3 tablespoons of vegetable oil, and 1 ½ tablespoons of cocoa powder. Melt them together in the microwave in 30-second intervals, stirring until smooth. In a separate bowl, whisk together 2/3 cup dark brown sugar, 2 large eggs, 2 teaspoons of vanilla extract, and ¼ teaspoon of salt. Mix this with the chocolate mixture, then fold in 3 tablespoons of all-purpose flour. Pour the brownie batter into the cooled crust and bake for about 15 minutes until the edges look set. Chill for at least 60 minutes, then refrigerate for an additional hour.

While the brownie layer cools, crush 6 additional Oreo cookies in a resealable bag. In a bowl, mix the crushed Oreos with 2 tablespoons of powdered sugar, 1 tablespoon of cocoa powder, a pinch of salt, and 2 tablespoons of melted butter until combined. Spread this mixture onto a baking sheet and bake at 325°F (163°C) for 10 minutes. Let the topping cool completely before adding it to the pie.

Melt 6 ounces of chopped milk chocolate using a double boiler or microwave, taking care not to overheat it. Heat 3 tablespoons of heavy cream in a small bowl, whisking in 2 tablespoons of cocoa powder until smooth. In a large mixing bowl, beat the remaining 1 cup of very cold heavy cream until soft peaks form. Gently fold in the melted chocolate and the cocoa mixture, achieving a fluffy mousse consistency. Spread the mousse evenly over the chilled brownie layer.

Once the mousse layer is set, sprinkle the cooled topping over the mousse, distributing it evenly. For the best flavor, cover the pie with plastic wrap and refrigerate for at least 3-4 hours, or ideally overnight. This chilling time allows the layers to meld beautifully together, resulting in a rich and decadent Mississippi Mud Pie that is ready to impress your guests.