Combine egg whites, granulated sugar, and salt in the heatproof bowl of a stand mixer. Whisk to combine.
Place the bowl over a pot of simmering water and continue to whisk the mixture. Continue to gently heat and whisk the mixture until the sugar granules are dissolved. A great way to check is to rub the mixture between two fingers to see that the mixture is smooth.
Attach the bowl to the stand mixer fitted with the whisk attachment. Whip the mixture on low for a minute and then turn the mixer up to medium-high speed. Continue whipping the egg white mixture until it is light, fluffy, and maintains stiff peaks. The whipping should help the meringue cool down. If you touch the side of the bowl, it should only feel slightly warm or be at room temperature.
With mixer on medium-low speed, add the butter one tablespoon at a time, mixing for a few seconds in between each addition. Scrape the sides and bottom of the bowl and continue to mix to ensure all of the ingredients are well combined.
After the butter is added, add in the vanilla extract and mix on medium speed until combined.
If desired (and highly recommended) switch to the paddle attachment and mix the buttercream on low speed for 2 minutes to remove the air bubbles from the buttercream.