In small saucepan, beat together milk, eggs, and sugar. Cook over medium heat, stirring constantly. (Or, place ingredients in microwave safe bowl and beat. Cover and cook in microwave on medium power for 1 minute. Beat with whisk and continue cooking at 20 second intervals, beating after each interval.)
Continue cooking until custard mixture reaches 160 degrees F or until it lightly coats the back of a metal spoon. Stir in vanilla. Cover and cool completely in refrigerator.
Place custard mixture in quart-sized zip closure freezer bag and seal. (Reinforce closure with duct tape, if desired.) Open gallon-sized zip closure bag; add ice, rock salt and sealed quart-sized bag filled with custard mixture. Seal gallon-sized bag and massage for 8-10 minutes.
Once frozen, leave custard in bag of ice for 5-10 minutes and allow ripening, if desired. Remove quart-sized bag of custard, open and enjoy!
Variations: You may substitute liquid pasteurized eggs for shell eggs. If doing so, the custard does not need to be cooked prior to freezing.