In the bowl of an electric mixer, combine the egg whites and vanilla and beat until frothy.

On low speed, add the sugar gradually until completely incorporated.

Increase speed to high and beat until the mixture forms stiff, glossy peaks, about 5-7 minutes. (Beating the icing is key to a lovely royal icing- this step makes your icing fluffy, stiff, AND white!)

For my cookies I kept half of the icing at this stiff consistency, and watered down the other half to form a flooding icing. To water down, mix in 1 tsp of water at a time until the desired consistency is reached. (Test flooding icing by dabbing a bit of icing on a flat surface. If it loses its shape after a 10 seconds,

Make ahead: store in the refrigerator for up to 3 days.