Beat powdered sugar and Just Whites in a large bowl with mixer on low speed until combined. Add 1/3 cup water; beat until blended. Increase speed to high; continue to beat 8 minutes or until icing is very thick. Icing can be tinted with paste or gel food colors, as desired. Pipe onto cookies, or thin with a bit more water for spreading.
To pipe icing: Use a pastry bag fitted with a plain tip, or fill a zip-top plastic bag with icing and snip off 1 of the corners. Pipe icing along the edges of the cookies or in a pattern you like. Immediately sprinkle with decorating sugar or edible glitter, if desired.