Bring a few inches of water to a boil in a saucepan that can hold a heatproof stand mixer's bowl above the water. Add the egg whites, sugar, cream of tartar and salt to the bowl and whisk together by hand. Set the bowl above the boiling water and whisk until the mixture is warm to the touch and the sugar dissolves, 1 to 2 minutes. Transfer to the stand mixer fitted with the whisk attachment and beat at medium-high speed until the whites are cooled and form a stiff meringue, about 5 minutes. If not using immediately, cover and store at room temperature.