1. Combine sugar, egg whites, and salt in bowl of stand mixer; place bowl over pan of simmering water. Whisking gently but constantly, heat mixture until slightly thickened, foamy, and registers 150°F on instant-read thermometer, 2 to 3 minutes.

2. Place bowl in stand mixer fitted with whisk attachment. Beat mixture on medium speed until consistency of shaving cream and slightly cooled, 1 to 2 minutes. Add butter, 1 piece at a time, until smooth and creamy. (Frosting may look curdled after half of butter has been added; it will smooth with additional butter.) Once all butter is added, add vanilla and peanut butter; mix until combined. Increase speed to medium-high and beat until light, fluffy, and thoroughly combined, about 30 seconds, scraping beater and sides of bowl with rubber spatula as necessary.

Makes about 2 1/4 cups