From On Cooking: A Textbook of Culinary Fundamentals (2nd Edition)

6 oz powdered sugar
1 egg white
1/4 tsp lemon juice

Sift sugar and set aside.

Place egg white and lemon juice in a stainless steal bowl.

Add 4 ounces of sugar and beat with an electric mixer until blended. The mixture should fall from a spoon in heavy globs. If it pours, it is too thin and will need the remaining 2 ounces of sugar.

Once the consistency is correct, continue beating for 3-4 minutes. The icing should be white, smooth and thick enough to hold a stiff peak. Food coloring can now be added as desired.

Cover the icing with a damp towel and plastic wrap to prevent it from hardening.