Stir in a small heavy saucepan until the sugar is dissolved, then bring to a boil over high heat:

Reduce the heat to low, cover, and simmer to allow the steam to wash down any sugar crystals on the sides of the pan. In the meantime, beat in a stand mixer with the whisk attachment at medium-high speed until stiff:

Turn the mixer down to the lowest speed and continue to beat. Uncover the syrup and cook to 235° to 240°F, the soft-ball stage. Beating the whites at high speed, add the syrup in a slow, thin stream down the side of the bowl. Avoid pouring the hot syrup onto the spinning whisk. Add:

Continue to beat at high speed until the icing has cooled, 7 to 10 minutes. Reduce the speed to medium and beat 1 more minute. Use at once.