Combine egg whites and sugar in a heat-safe bowl. Set the bowl over a pot of simmering water. Whip mixture until the temperature reaches 120 degrees F (49 degrees C) on a digital or instant thermometer and sugar is dissolved, about 5 minutes.
Remove egg white mixture from heat. Continue to whip until soft peaks form and the meringue temperature has come down to 99 degrees F (37 degrees C) or lower, 4 to 7 minutes.
Add butter to the meringue, 2 tablespoons at a time, whipping until incorporated and fluffy. Scrape the sides of the bowl and keep mixing until smooth, 5 to 8 minutes. Add vanilla extract and whip until incorporated.
