Place the egg whites and sugar in a spotlessly clean heatproof bowl (the bowl of your stand mixer if you have one) and place the bowl over a pan of gently simmering water, make sure that the base of the bowl doesn't touch the water. Gently whisk the eggs (with a hand whisk, not an electric one) until the sugar has dissolved, if you rub the mixture between your fingers it should feel smooth with no hint of graininess. It should measure about 60C/140F on a thermometer.
Remove the bowl from the heat and whisk with an electric mixer/stand mixer on high until the egg whites have formed a stiff, glossy meringue and the mixture is completely cold, the base of the bowl should feel cool to the touch, this will take about 10 minutes.
Turn the mixer down to low and gradually add the butter, a tablespoon at a time, mixing to incorporate after each addition. The mixture may curdle or begin to look soupy, just keep on whisking and it will come back together.
Once all of the butter has been incorporated and the buttercream is smooth and silky whisk in the vanilla extract and salt.