In standing mixer fitted with whisk attachment, beat butter at medium-high speed until smooth, about 20 seconds. Add confectioners sugar and salt; beat at medium-low speed until most of the sugar is moistened, about 45 seconds. Scrape down bowl and beat at medium speed until mixture is fully combined, about 15 seconds; scrape bowl, add vanilla and beat at medium speed until incorporated, about 10 seconds, then reduce speed to low and gradually beat in chocolate. Increase speed to medium-high and beat until light and fluffy, about 4 minutes, scraping down bowl once or twice.

Makes 1 1/2 cups, enough for 12 cupcakes