Prepare and bake the brownies according to the box directions in a 13"x9" pan. Let the brownies cool.
Mix the powdered sugar, butter, and peanut butter. Spread the peanut butter frosting over the brownies. Let the peanut butter and brownies continue to chill for one hour.
In a small saucepan, pour the whipping cream. Over medium heat, warm the cream until it just begins to bubble. Remove from the heat and pour the dark chocolate into the cream. Stir to melt chocolate. Let sit for 5-10 minutes to cool slightly. Pour over the peanut butter frosting.