Beat the powdered sugar, milk and egg whites with a mixer in a large bowl until thick and glossy. Add more powdered sugar if the icing seems too thin (the size of the eggs can affect consistency). Just play with it until it's right. The icing should be stiff enough to "stand still" when piped and not run.
Consumption of raw or undercooked eggs, shellfish and meat may increase the risk of foodborne illness.
Recipe courtesy of Ree Drummond