1 small egg white
150g icing sugar
Pour all ingredients into a bowl and whisk vigorously for several minutes until a thick cream, between the Italian meringue and melted marshmallow(if you've never tried this, you don't know what you're missing).
I recently found at Tesco that you could buy all-in-one Royal Icing sugar. That's quite useful if you have to make a lot. But this one won't have any special taste. So instead of water, pour some extract!