Preheat oven according to package instructions and line an 8x8 pan with parchment paper.
Mix brownie batter according to package instructions and pour half of the batter into the prepared baking pan.
Arrange unwrapped Reese's cups in the pan on top of the batter, pour remaining batter over the Reese's cups and bake according to package instructions, generally 35 to 45 minutes.
Remove and allow to cool before frosting.
Frosting:
Cream Butter using a paddle attachment.
Add vanilla and peanut butter and beat until well mixed.
Add confectioners' sugar a cup at a time, alternating with heavy cream.
Whip on medium-high speed for 2-3 minutes until frosting is light and fluffy.
Frost brownies and sprinkle with chopped Reese's cups.