Whip the Whites: Add the egg whites to the bowl of the stand mixer and whisk on medium speed until the whites are frothy. Add the cream of tartar and continue to whip on medium speed until the egg whites form soft peaks.

Add the Sugar: With the mixer off, add one cup of the powdered sugar to the whites. Turn the mixer on, slowly going from low speed and working up to high speed. Once the sugar is incorporated, add another cup while the mixer is running. Continue to add the powdered sugar in batches until you have added all four cups.

Correct Consistency: Add the vanilla extract and mix it into the icing. If you want to use the icing to cover cookies completely, it should be slightly runny. If you are doing fine decoration, the icing should be stiff. Add a little water to loosen up the icing or add a little more powdered sugar to make it thicker.

Color: Add any food coloring you’d like then the royal icing is ready to use. It is perfect for decorating sugar cookies and will harden on the cookie after about 2 hours.