Instructions

1. Place reserved egg whites in the bowl of an electric mixer with cream of tartar and beat at high speed until stiff peaks form. 
2. Add cold water and sugar to a heavy bottom saucepan and place over medium heat. Bring the mixture to a boil, scraping any sugar from the sides of the pan.
3. Cook at a boil until mixture reaches 235°.
4. Remove from heat and slowly pour syrup into the egg whites with the mixer running on high speed. 
5. Continue mixing for about 5 - 7  minutes until icing is fluffy. 
6. Spread onto baked sugar cookies and add sprinkles if desired. 
7. Place iced sugar cookies in a single layer on a wire rack or baking sheet to dry completely. This can take an hour or more but the icing will dry completely and then cookies can be stacked.