In the bowl of a stand mixer fitted with the paddle attachment, beat butter at medium speed until creamy, 3 to 4 minutes.
In another large bowl, whisk together confectioners’ sugar and cocoa. Gradually add sugar mixture to butter, beating until smooth. Add milk, 1 tablespoon (15 grams) at time, beating until frosting is a spreadable consistency. Beat in vanilla.
Recipe by Bake from Scratch at https://staging.bakefromscratch.com/chocolate-buttercream-cake/