In the bowl of a stand mixer fitted with a paddle attachment, combine egg white with half the powdered sugar. Mix at low speed until sugar disappears into a paste, then gradually sprinkle in the rest. Increase speed to medium and beat until smooth and light, about 2 minutes. If too stiff, thin icing with a few drops of water. Use immediately; keep covered tightly in plastic to prevent icing from drying in bowl. Royal icing can be kept in an airtight container at room temperature up to 12 hours.
