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 crystals that may have formed 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 until a thermometer reads 235° to 240°F, the soft-ball stage. Beating the egg 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. Beat for 10 minutes, until cooled. Add:

Beat the icing to a spreading consistency and use immediately.