1. Combine eggs, sugar, vanilla, and salt in bowl of standing mixer; place bowl over pan of simmering water. Whisking gently but constantly, heat mixture until thin and foamy and registers 160 degrees on instant-read thermometer.

2. Beat egg mixture on medium-high speed with whisk attachment until light, airy, and cooled to room temperature, about 5 minutes. Reduce speed to medium and add butter, one piece at a time. (After adding half the butter, buttercream may look curdled; it will smooth with additional butter.) Once all butter is added, increase speed to high and beat 1 minute until light, fluffy, and thoroughly combined. (Can be covered and refrigerated up to 5 days.)