Add the powdered sugar and meringue powder to a stand mixer fitted with the whisk attachment.
Slowly stream in the warm water, beating on high speed for 2 minutes.
Mix in the vanilla extract.
To test if it’s ready, lift up the whisk. The icing should fall back into the bowl in a ribbon and should re-incorporate into the icing in the bowl within 5-10 seconds.
If it’s too thin, continue mixing on high speed until it reaches this consistency. The longer you beat royal icing, the thicker it becomes. If it’s too thick, you can always beat in a splash of water if needed.
Whisk/beat in gel food coloring as desired.
This icing takes about 2 hours to dry completely once applied on cookies (or longer if it’s a thick layer).
Notes
Powdered sugar: It's important to use powdered sugar (confectioners sugar) not granulated sugar.
Meringue powder: Meringue powder is a mixture of pasteurized egg whites, sugar, and other ingredients. It is commonly sold near cake decorating supplies, and it can also be ordered online.
Warm water: Warm water helps everything to mix together nicely.
Clear vanilla: Using clear vanilla will keep the royal icing white. If you use a different flavoring or pure vanilla extract, it may change the color of the icing.
Gel food coloring: Gel food coloring works well because it doesn't affect the consistency of the icing the way that regular food coloring can.
Yield: This recipe makes 3 cups of royal icing. The number of servings will vary depending on how much frosting you use per cookie.