In a large bowl or the bowl of a stand mixer fitted with a whisk attachment, add the egg whites. Whip (using a stand mixer, hand mixer or whisk) the egg whites until very frothy. If mixing by hand, this may take up to 5 minutes.
Add the sugar and vanilla and continue to mix until well combined and the icing is cohesive and glossy (see note). Transfer to a piping bag or add colouring as desired.
Video
Notes
To adjust the consistency, simply add more sugar to reach the level you desire.
Make Ahead
Royal icing can be made up to 1 week in advance. Transfer to an airtight container, press a sheet of plastic wrap directly against the surface of the icing to prevent a skin from forming, then seal and refrigerate. Allow to come to room temperature before piping.