Bake the brownie base: Preheat the oven to 350°F. Line the bottom of a 9-inch springform pan with parchment paper. Prepare the brownie batter according to the package directions and spread it evenly in the pan.
Bake: Bake for about 35 to 40 minutes, or until the brownie is set but still slightly fudgy in the center. Cool completely before adding the next layer.
Make the peanut butter filling: Beat the heavy whipping cream until it begins to thicken. Gradually add the powdered sugar and continue beating until stiff peaks form.
In another bowl, beat the softened cream cheese and peanut butter until smooth. Add the brown sugar and vanilla extract, then mix until well combined.
Combine the filling: Gently fold the whipped cream into the peanut butter mixture. Fold in the crushed peanut butter cookies until evenly distributed.
Add the cheesecake layer: Spread the peanut butter filling over the cooled brownie base. Smooth the top with a spatula.
Prepare the gelatin: Sprinkle the gelatin over the cold water in a small microwave-safe bowl. Let it stand for a few minutes until softened. Microwave briefly, just until the gelatin becomes liquid. Do not overheat it.
Slowly add the melted gelatin while mixing. Continue whipping until the cream reaches stiff peaks.
Slowly add the melted gelatin while mixing. Continue whipping until the cream reaches stiff peaks.
dd the cookie mousse: Gently fold in the crushed peanut butter cookies. Spread the cookie mousse evenly over the peanut butter layer.
Chill: Cover the pie and refrigerate for at least 4 hours, preferably overnight, until the mousse is firm.
Decorate: Before serving, whip the remaining heavy cream with the powdered sugar until stiff peaks form. Pipe or spoon the whipped cream around the edge of the pie.
Place halved peanut butter cookies around the whipped cream and sprinkle crushed cookies over the top.
Serve: Carefully remove the pie from the springform pan. Slice with a sharp knife and serve chilled.
