In a small saucepan, combine the granulated sugar with the water and bring to a boil over moderately high heat, stirring constantly until the sugar has dissolved, 2 to 3 minutes.
In a large bowl of a standing electric mixer, beat the egg whites with the pinches of cream of tartar and salt until soft peaks form.
With the mixer at medium speed, carefully pour the hot sugar syrup into the egg whites. Continue beating the frosting until cool and billowy, about 5 minutes. Immediately spread the frosting on the cupcakes and top as desired.