In bowl of an electric mixer, beat powdered egg white, water and cream of tartar. Add confectioners' sugar and mix on medium speed until blended. Increase speed to high and beat until icing is thick and glossy, about 5 minutes. Add more water as needed to thin to desired consistency for piping or spreading. Use immediately. (Or refrigerate in a covered plastic container. Beat icing again to soften before using.)
I made these and they were hard as rocks. I thought when I was making them that the flour to butter ratio seemed wrong. It was.