Combine 1 cup milk and cornstarch in top of a double boiler, stirring until well blended.
Add remaining 3 cups milk and sweetened condensed milk; stir well. Cook over boiling water, stirring constantly, until mixture thickens and is smooth.
Beat egg yolks slightly. Gradually stir one-fourth of hot milk mixture into yolks; add to remaining hot milk mixture, stirring constantly. Cook over boiling water, stirring constantly with a metal spoon, until mixture thickens and coats the spoon.
Remove from heat, and stir in vanilla and salt. Set custard aside and let cool to room temperature.
Beat egg whites (at room temperature) until stiff peaks form. Gently fold egg whites into cooled custard mixture; chill thoroughly. Spoon custard into individual serving bowls to serve.
