For the Chocolate Buttercream Frosting - Once the brownies are completely cooled, make the frosting. To a medium size bowl, add melted butter and cocoa powder. Whisk together until combined.
To the bowl, add milk, powdered sugar and vanilla extract. Beat with a mixer until smooth and creamy.
The frosting is soft and creamy. For a thicker frosting add more powdered sugar 1/4 cup at a time until it reaches the desired consistency.
To the completely cooled brownies that are still in the pan, spread the frosting evenly on top. On top of the frosting, evenly add the cut pieces of butterfinger candy.
Chill in the refrigerator for at least 2 hours (while the brownies are still in the pan) before cutting the brownies.
Cutting the Brownies - Grab ahold of the sides of the parchment paper, use them as a "sling" and lift the brownies out of the pan.Set the brownies (with the parchment paper still attached) onto a large cutting board.
Before cutting the brownies, warm a large knife with hot tap water, dry the knife and make a cut. Wash off, re-warm and dry the knife after each cut.
Line a large plate or platter with fresh parchment paper and place each brownie onto it for serving.
They're best when served as soon as possible after making them because the butterfinger candy on top of the brownies may become a little sticky.