Over a double boiler, heat the whites and granulated sugar and salt, whisking constantly, until all the sugar is dissolved. Heat this slowly!! You don?t want scrambled eggs!
Place the mixture in a stand mixer and whip on high until a peak forms and the mixture is cool, about 5 minutes.
Turn the mixer to medium speed and add the softened butter a little at a time. Remember, depending on your area, you may need more or less butter.
Add the powdered sugar and vanilla and mix.
Tip: In warmer climates/seasons you will want to change the amount of butter to 4 sticks (2 cups) and also add 1 1/2 cups of vegetable shortening to the buttercream. This will give more stability when the weather is hot.