In a standing mixer fit with the whisk attachment on medium-high speed, add the egg whites and vanilla and beat until foamy.
Reduce mixer speed to low and add the powdered sugar until just incorporated and shiny. Beat on high for 5 to 7 minutes or until the mixture is thick and glossy with stiff peaks.
Add food coloring if desired, then transfer to piping bags or spread with a spatula. If using sprinkles or sugar, add quickly before the frosting hardens.
Egg whites: Cartons of pasteurized egg whites are available at most grocery stores. Or swap in meringue powder. (Check out Recipe Tips and Variations for how to make Royal Icing with meringue powder instead.)
Vanilla: Use clear vanilla if you are worried regular vanilla may tint your icing.
Storage: Place in an airtight container and refrigerate for up to 3 days.
Make ahead: Royal Icing can be made up to 3 days ahead, if stored in an airtight container in the refrigerator. It freezes well, too. After your cookie decorating project, any icing leftovers can be frozen in zip-top plastic bags for up to 2 months. To thaw the icing, allow it to come up to room temperature, then give it a gentle stir before using.
Freezer: Transfer any extra Royal Icing to a zip-top plastic bag and freeze for up to 2 months. To thaw the icing, allow it to come up to room temperature, then give it a gentle stir before using.