In the bowl of a stand mixer fitted with the paddle attachment, beat butter and salt at low speed until smooth.

In a large bowl, sift together confectioners’ sugar and cocoa. With mixer on low speed, add sugar mixture, about 1 cup at a time, to butter mixture alternately with cream, about 1 tablespoon (15 grams) at a time, beginning and ending with sugar mixture, beating until smooth after each addition and stopping to scrape paddle and bottom and sides of bowl. Beat in vanilla. Increase mixer speed to medium, and beat until fluffy, 2 to 3 minutes. Use immediately.