In a small saucepan, bring a small amount of water to a boil. Reduce the heat to simmer.
In medium heat-proof bowl (I used my stand mixer bowl), whisk together egg whites and sugar and place it over the saucepan with simmering water. (This is a make-shift double boiler. Make sure the bottom of the top bowl doesn't touch the water.)
Cook the egg white mixture until sugar is completely melted and it reaches 160°F (70°C) stirring continuously, about 5 minutes. (This's my favorite thermometer!)
Now, remove the bowl with egg white mixture from heat. Then whisk it on medium speed for a minute, then slowly increase the speed to medium high and beat until stiff peaks form and the mixture is cool to touch, about 10 minutes. (It may take longer with a hand-mixer.)
Once the meringue reached stiff peaks, scrape the side of the bowl with a spatula and add salt.
Turn on the mixer on medium speed and start adding butter, one tablespoon at a time. Make sure the butter is fully incorporated before adding the next piece. (NOTE: It's normal if buttercream starts to curlde half way. Keep adding the butter!)
Keep whisking the buttercream until smooth, light and fluffy, about 10 minutes.
Now, switch to a paddle attachment and add vanilla bean. Beat the buttercream until well combined, about a minute.