Begin by preheating your oven according to the instructions on your brownie mix. Prepare the brownie batter as directed on the box, using your favorite mix. Pour the batter into a greased 9-inch pie dish, spreading it evenly.
Bake the brownies according to the package instructions. Once baked, allow them to cool completely in the pie dish on a wire rack.
In a medium bowl, combine the peanut butter, chocolate chips, and butter. Melt the mixture together using a double boiler or microwave, stirring until smooth. Set aside to cool slightly.
In a separate bowl, whip the whipping cream until soft peaks form. Gently fold the whipped cream into the cooled peanut butter mixture until well combined.
Pour the peanut butter mousse over the cooled brownie base, spreading it out evenly. Allow the pie to chill in the refrigerator for at least 2 hours, or until the mousse is set.
In a small bowl, mix the crushed corn flakes with a drizzle of melted butter until the flakes are coated. Spread this mixture evenly over the set mousse layer.
Melt the semi-sweet chocolate with the milk in a microwave-safe bowl, stirring until smooth. Drizzle this chocolate over the corn flake topping and scatter marshmallows over the top.
Place the pie back in the refrigerator for an additional 30 minutes to set the chocolate drizzle. When ready to serve, slice your Peanut Butter Crunch Brownie Mousse Pie and enjoy!