Step 1: Prepare the crust. Combine cookie crumbs, melted butter, and granulated sugar in a medium bowl. Press firmly into the bottom of a 9-inch springform pan. Place in the freezer to chill while you prepare the filling.
Step 2: Make the peanut butter mixture. In a large bowl, beat the softened cream cheese and creamy peanut butter with an electric mixer until smooth and creamy. Gradually add the powdered sugar, vanilla extract, and salt, mixing until fully combined.
Step 3: Whip the heavy cream. In a separate, clean, cold bowl, whip the cold heavy cream until stiff peaks form. Be careful not to over-whip.
Step 4: Fold and combine. Gently fold about one-third of the whipped cream into the peanut butter mixture to lighten it. Then, fold in the remaining whipped cream until no streaks remain. Stir in the chopped peanut butter cups.
Step 5: Assemble and chill. Pour the peanut butter mousse mixture over the chilled crust in the springform pan. Smooth the top with a spatula. Cover loosely with plastic wrap and refrigerate for at least 4 hours, or preferably overnight, until firm.
Step 6: Garnish and serve. Once set, carefully remove the sides of the springform pan. Garnish with additional chopped peanut butter cups and chocolate shavings or a drizzle of chocolate sauce before slicing and serving.