Put sugar and water in small saucepan and set on stove.
Put egg whites in standing mixer fitted with whisk attachment and turn on medium.
Fill a small bowl with very cold water and place on stove next to working area.
Heat sugar mixture, stirring continually, until heat reaches 240°F or “soft ball” stage (about 2 to 3 minutes).
Simultaneously, once egg whites are foamy, add cream of tartar and salt and turn to high to form soft peaks.
Turn speed on mixer back to medium and carefully pour hot sugar mixture into egg white mixture.
Whisk at medium speed until bowl is cool to the touch. Then, turn speed to high until firm peaks form.
Place frosting in piping bag fitted with a large round tip and pipe onto cooled ganache.