In a large bowl, beat the powdered sugar, milk and egg whites with an electric mixer 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" and not run when piped.
Consumption of raw or undercooked eggs, shellfish and meat may increase the risk of foodborne illness.