Bring a few inches of water to a boil in a saucepan that can hold a mixer’s bowl above the water. Whisk the sugar, the egg whites, lemon juice, cream of tartar, and salt in the bowl by hand. Set the bowl above the boiling water and continue whisking until the mixture is hot to the touch and the sugar dissolves, about 1 to 2 minutes. Transfer the bowl to a standing mixer fitted with the whisk attachment and beat the whites at medium-high speed until they almost holds a stiff peak and are cool, about 10 minutes. Beat in the butter, a little at a time, until the icing is smooth and spreadable. (If the icing separates, just keep beating and it will come back together.)