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.