Heat the milk in double boiler to scalding point. Blend cornstarch, sugar and salt. Pour part of the milk slowly over dry ingredients stirring constantly, gradually add all of the hot milk. Return to double broiler and cook for 20 minutes, stirring to keep it smooth. Beat egg yolks, add a little of the hot mixture, stirring to prevent lumping. Add this to hot mixture in double boiler, stirring vigorously. Cook two minutes. Remove from heat, add lemon juice, rind and butter. Beat until smooth. Cool. Fold in stiffly beaten white of one egg. Allow mixture to become cool. Pour into baked shell. Cover with meringue made with the two remaining egg whites, salt and sugar. Bake in 350 degrees oven for 15 mintues or until lightly browned.