Insert the Butterfly whisk. Add the egg white and lemon juice to the Thermomix bowl and whisk for 40 sec/speed 3.
Remove the Butterfly whisk. Scrape down the bowl and add the icing sugar. Mix for 20 sec/speed 4. Scrape down the bowl then, repeat for 20 sec/speed 4.
Use as needed to decorate cookies, gingerbread etc. Add some colour with a few drops of food colouring.
Notes
Icing sugar: To make your own icing sugar, see my blog post on how to make Thermomix icing sugar from white sugar.StorageThis Royal Icing contains egg whites, so it must be stored in the fridge and consumed within three days. Bring it to room temperature before using it.