Place egg whites, salt, Rodelle Vanilla Extract, and sugar in a large mixing bowl, whisk thoroughly.
Place egg white mixture in a double boiler and heat water to a simmer on LOW to just warm the egg whites (the small amount of indirect warmth from the water underneath will help dissolve the sugar while NOT cooking the eggs).
Heat until sugar is dissolved, about 3-5 minutes.
Set egg white mixture aside and let cool.
With the paddle attachment on an electric mixer, cream butter on medium until creamy.
Slowly add the egg white mixture ½ cup at a time, scraping down sides as needed.
Change to the whisk attachment and whisk for 2-3 minutes until creamy and well incorporated.
Add powdered sugar 1 cup at a time, mixing on medium and then high in between.
Mix on high for 1-2 minutes until frosting is light and fluffy.
If the frosting is too heavy, add additional powdered sugar (1/4 cup at a time) until fluffy.
