Preheat your oven according to the instructions on the brownie mix box. Prepare the brownie batter as directed, pouring it into a greased 9-inch pie dish.
Bake the brownies for 20-25 minutes as specified on the box. Allow them to cool completely in the pie dish.
In a large mixing bowl, beat the softened cream cheese with the creamy peanut butter and confectioners' sugar until smooth.
In another bowl, whip the cold heavy whipping cream until stiff peaks form.
Gently fold the whipped cream into the peanut butter mixture using a spatula.
Pour the peanut butter mousse over the cooled brownie base, spreading it evenly.
Cover the pie with plastic wrap and refrigerate for at least 4 hours, or overnight if possible.
Before serving, garnish with whipped cream and shaved chocolate if desired. Slice and enjoy!
