Sift sugar and cream of tartar into a large mixing bowl. Add egg whites and vanilla; beat with an electric mixer at low speed until mixed. Increase speed to high and beat until icing has thickened and holds peaks, 8 to 10 minutes.Advertisement
Decorate cookies (see Cookie-Decorating Tips, with Holiday Cookie Cutouts recipe). While working, keep the icing covered with a damp towel to prevent it from drying out. Let cookies stand until icing has set, about 30 minutes.
Ingredient Note: To avoid health concerns about uncooked eggs, use pasteurized egg whites. You can find both refrigerated cartons of pasteurized egg whites and convenient pasteurized dried egg whites in most supermarkets.
Decorate on parchment: When adding finishing touches to your cookies or cakes, place them on a large sheet of parchment paper before you decorate. The paper catches the excess, making cleanup a breeze.