In the bowl of a stand mixer fitted with the whisk attachment, beat butter at medium speed until creamy, 3 to 4 minutes. In a large bowl, sift together confectioners’ sugar, cocoa, and salt. Gradually add confectioners’ sugar mixture to butter, beating until combined. Stir in enough buttermilk until frosting reaches a spreadable consistency
Recipe by Bake from Scratch at https://staging.bakefromscratch.com/devils-food-cake-with-chocolate-buttermilk-frosting/