Royal icing hardens when it dries, making it particularly useful when decorating sugar cookies.
Combine dried egg whites (or meringue powder), water, cream of tartar and lemon (or almond) extract in a large bowl. Add sugar and beat with an electric mixer on medium speed until stiff peaks form, 6 to 12 minutes.
Add water if needed to adjust icing consistency for piping or spreading. Add food coloring, if desired. Keep a damp cloth or paper towel directly on the icing while you work, to keep it from drying out.
Make Ahead Tip: If not using immediately, cover icing with a damp cloth and plastic wrap and store in the refrigerator for up to 3 days. Beat again before using.