Soften gelatin by stirring it in the cold skim milk in the top of a double boiler; add hot milk and salt. Place over boiling water and stir until gelatin is entirely dissolved. Pour hot mixture slowly into egg yolks,stirring constantly. Return to top of double boiler over hot, but not boiling, water stirring constantly until mixture coats a metal spoon. Remove from heat. Add vanilla and sugar, stirring to dissolve completely. Cool and chill until slightly thicker than unbeaten egg whites. Fold into beaten egg whites. Turn into a large mold or individual ones. Chill until firm.