Place reserved egg whites in the bowl of an electric mixer with cream of tartar and beat at high speed until stiff peaks form. 

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.

Cook at a boil until mixture reaches 235°.

Remove from heat and slowly pour syrup into the egg whites with the mixer running on high speed. 

Continue mixing for about 5 - 7  minutes until icing is fluffy. 

Spread onto baked sugar cookies and add sprinkles if desired. 

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.