Step 1: Prepare the Oreo Crust

1. Crush the Oreos:  

   Place 24 Oreo cookies (with filling) in a food processor and pulse until they become fine crumbs. If you don’t have a food processor, place the Oreos in a sealed ziplock bag and crush them with a rolling pin until fine.

2. Mix with Butter:  

   In a large bowl, combine the Oreo crumbs with 1/2 cup melted butter. Stir until the crumbs are evenly coated with butter.

3. Form the Crust:

   Press the mixture into the bottom of a 9-inch springform pan. Press it down firmly using the back of a spoon or your fingers until it’s compact and even.

4. Chill:

   Refrigerate the crust for at least 15 minutes to firm it up.

Step 2: Prepare the Peanut Butter Mousse Layer

1. Whip the Cream Cheese and Peanut Butter: 

   In a large mixing bowl, beat together 8 oz softened cream cheese and 1 cup peanut butter until smooth and creamy.

2. Add Powdered Sugar and Vanilla: 

   Add 1 cup powdered sugar and 1 teaspoon vanilla extract. Beat until the mixture is well combined.

3. Whip the Heavy Cream:  

   In a separate bowl, whip 1 cup cold heavy whipping cream until stiff peaks form (you can use a hand mixer or stand mixer).

4. Fold in the Whipped Cream: 

   Gently fold the whipped cream into the peanut butter mixture until fully combined. Be careful not to deflate the whipped cream too much.

5. Spread the Mousse:

   Once the crust has chilled, spread the peanut butter mousse layer evenly over the Oreo crust. Smooth out the top with a spatula.

6. Chill: 

   Place the pan back in the refrigerator and let it set for at least 1 hour, or until the mousse is firm.

Step 3: Prepare the Chocolate Mousse Layer

1. Melt the Chocolate:  

   Melt 8 oz dark chocolate in a heatproof bowl over simmering water (double boiler method). Stir until smooth and fully melted. Let it cool slightly.

2. Whip the Heavy Cream:  

   In another bowl, whip 1 cup cold heavy whipping cream until stiff peaks form. If you prefer a sweeter chocolate mousse, add 1 tablespoon powdered sugar while whipping.

3. Combine the Chocolate and Cream: 

   Gently fold the cooled, melted chocolate into the whipped cream. Stir until fully combined and smooth.

4. Layer the Chocolate Mousse:  

   Once the peanut butter mousse has set, carefully spread the chocolate mousse on top, smoothing it out to cover the entire cake.

5. Chill Again:

   Refrigerate the cake for at least 4 hours, or overnight for best results. The longer it chills, the better the flavors will meld together.

Step 4: Garnish and Serve

1. Whip Some Cream:  

   For an extra indulgent touch, whip some additional heavy cream and pipe or spread it on top of the chocolate mousse layer.

2. Add Toppings:  

   Garnish with crushed Oreos, chopped peanuts, mini chocolate chips, or a drizzle of melted chocolate or peanut butter.

3. Slice and Serve:  

   To slice the cake, run a knife under hot water and dry it off between cuts for clean slices. Serve chilled and enjoy the rich, creamy layers of chocolate peanut butter mousse!