Place the Butter (1 cup) in the bowl of a stand mixer fitted with the paddle attachment.
Mix on a high speed until the butter is pale and fluffy, about 5 minutes.
Add the Powdered Confectioners Sugar (2 cup) and Dark Cocoa Powder (1/2 cup) and mix until it just comes together.
Add the Milk (1/4 cup) and mix on a high speed until light and fluffy.
Frost your cake and decorate as desired. Serve and enjoy!