1 envelope unflavored gelatin (1 scant tablespoon)
⅓ cup plus ¼ cup cold water
1 cup sugar
In a mixing bowl, sprinkle gelatin over ⅓ cup cold water. Allow gelatin to soften,about 5 minutes.
Heat remaining ¼ cup water and the sugar in a saucepan over medium-high, stirring until sugar is dissolved. Stop stirring; clip a candy thermometer onto side of pan. Boil syrup until temperature reaches the soft-ball stage (238º F), brushing down sides of pan with wet pastry brush to prevent sugar crystals from forming. Remove from heat; add syrup to softened gelatin. Whisk mixture by hand to cool, about 1 minute, then use an electric mixer to whisk on medium-high speed until soft, glossy (but not dry) peaks form, 8 to 10 minutes. Use immediately, as frosting will harden quite quickly.