In a medium bowl, beat butter with hand mixer fitted beater attachments at medium speed until smooth and creamy, about 1 minute. Add cocoa, and beat at low speed until combined. With mixer on low speed, add confectioners’ sugar, about ½ cup (60 grams) at a time, alternately with buttermilk, about 1 tablespoon (15 grams) at a time, beating until combined. Increase mixer speed to medium, and beat until slightly thickened, about 1 minute.