Bake brownies according to package instructions and allow to cool completely.
In a large bowl, combine butter and peanut butter with a hand mixer until smooth. Add in vanilla and powdered sugar and stir until well combined. Add in milk and beat until light and fluffy. Refrigerate for 15 or so minutes to allow to thicken just a tad
In a separate bowl, add 5 tbs and chocolate chips and microwave in 20 second intervals until completely melted
Remove peanut butter topping from refrigerator and pour onto brownies in a smooth layer
Pour melted chocolate over peanut butter topping in a smooth layer and refrigerate until topping has hardened completely.
Slice brownies and serve with halved mini peanut butter cups on top