4 egg whites
6 cups powdered sugar
2 tsp cream of tartar
2 tsp vanilla extract
In the bowl of a stand mixer fitted with the whisk attachment, beat the egg whites with cream of tartar and vanilla until frothy.
Slowly add in the powdered sugar. Beat until everything comes together and is smooth. If it seems really thick add some water. A drop at a time. You want it to be thick enough so you can pipe it or spoon it on the cookies but it won't run everywhere.
Divide the frosting into bowls and add your choice of food coloring. Now you're ready to decorate cookies!
After all the cookies are decorated, let the icing dry for at least 8 hours.