Preheat oven to 300 degrees. With an electric mixer, beat butter until creamy. Add flour, sugar, and cocoa; mix just until combined. (Chill dough in the refrigerator 10 minutes if it is too soft to handle.)
Pat dough into an 8-inch round cake pan; press edges down with the tines of a floured fork. Bake until firm, 30 minutes. Immediately score into eight wedges; cool completely. Turn out of pan; break wedges apart. Decorate with icing .