In a large bowl, combine the egg whites and sugar. Set the bowl over (but not touching) simmering water on a saucepan to heat the mixture. Whisk constantly until the sugar has completely dissolved and the mixture is very warm but can be touched (about 70° C on an instant-read thermometer). After about 2 minutes, remove the bowl from the saucepan.
Using an electric mixer on high speed, beat the egg white mixture until it is fluffy, cooled to room temperature, and holds stiff peaks (the mixture should not look dry). This process may last for about 6 minutes.
With the mixer on medium-low speed, add the salt and the butter, a few pieces at a time, beating well after each addition. If the frosting appears to separate or is very liquid after all the butter has been added, continue to beat on high speed until it is smooth and creamy for about 3-5 minutes more. Add Mangoness and beat until combined. Scrape down the sides of the bowl as needed.