In the bowl of an electric mixer, using the whisk attachment, stir the egg whites and lemon juice together until combined. (Do not whip egg whites until fluffy. Leave them liquid.)

Add powdered sugar by the 1/2-cup measure, beating thoroughly between additions, until a stiff icing forms. The icing should leave stiff peaks that don't fall over or lose their shape when you pull up the beater. But i    t should still be flexible enough to pipe or spread easily, though.