Combine egg whites and sugar in a stainless steel bowl and place over a pot of simmering water (the mouth of the pot should be large enough to securely hold the bowl, alternately you can do this in the top of a double boiler)

Whisk constantly with a wire whip or spatula until the sugar completely dissolves (you can test this by rubbing some of the meringue between your fingers, you should not feel any sugar grains, it should be completely smooth)

Transfer the mixture to the bowl of a stand mixer fitted with the whisk attachment and whip at high speed until stiff peaks have formed and the bowl is cool to the touch. Add the salt.

Add the softened butter a bit by bit waiting till the previous addition is combined before continuing. (I like to switch over to the paddle attachment at this point simply because it’s easier to get the finished buttercream off the paddle than the whisk)

While mixing, the buttercream may appear to curdle but keep whipping and it will come back together into perfect beautiful buttercream, if it seems hopeless, place it briefly over some simmering water and whip it again.