Put heavy frying pan with close fitting cover on to heat. Separate eggs and beat yolks until thick, then add cornstarch. Beat whites stiff, add to yolk mixture and finally add milk and salt and pepper to taste. Melt butter in hot pan, pour in mixture. Cover closely and cook over medium heat for 6 or 7 minutes. Remove cover and with spatula fold omelet in half. Do not lift cover until cooked.