In a large saucepan, beat together eggs, sugar and salt, if desired. Stir in 2 cups of the milk. Cook over low heat stirring constantly, until mixture is thick enough to coat a metal spoon with a thin film and reaches 160 degrees F. Remove from heat. Stir in remaining 2 cups of milk and vanilla. Cover and refrigerate until thoroughly chilled, several hours or overnight. Just before serving, pour into bowl or pitcher. Serve immediately.
