In standing mixer fitted with whisk attachment, beat butter and peanut 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 heavy cream, and beat at medium speed until incorporated, about 10 seconds, then increase speed to medium-high and beat until light and fluffy, about 4 minutes, scraping down bowl once or twice.

Make 1 1/2 cups, enough for 12 cupcakes Any of the buttercream frostings (see related recipes) can be made ahead and refrigerated; if refrigerated, however, they must stand at room temperature to soften before use. If using a hand-held mixer, increase mixing times significantly (at least 50 percent). This recipe can be doubled to make enough for a two-layer cake.