In a *standing mixer fitted with the whisk attachment, beat the butter at medium-high speed until smooth, about 20 seconds. Add the cocoa powder and beat mixture until combined.

Add powdered sugar and salt; beat at medium-low speed until most of the sugar is moistened, about 1 minute. Scrape down the bowl and beat at medium speed until mixture is fully incorporated, about 30 seconds; scrape bowl, add vanilla and heavy cream or milk, and beat at medium speed until incorporated, about 20 seconds, then increase speed to medium-high and beat until light and fluffy, about 2 minutes, scraping down bowl once or twice.

Use to frost cupcakes or cakes. (24 cupcakes)