In a stand mixer with a paddle attachment on medium speed or using a wooden spoon and lots of muscle, beat butter until smooth and creamy. Reduce speed to low and add confectioners’ sugar, cocoa, and vanilla, scraping down sides of the bowl with a spatula as you go. Add milk as needed until frosting reaches preferred spreading consistency. (For thicker frosting, add less than a cup of milk. To thin out your frosting, add more milk. The choice is yours!)