Mix cornstarch with half cup milk until smooth. Add additional 1.5 cups milk and cook over very low heat until thick and smooth, stirring constantly to avoid lumps. Blend sugar, eggs, salt and evaporated milk in electric mixer bowl. Add hot cornstarch mixture and beat well. Add whipping cream, remaining 4 cups milk, chocolate and vanilla. Pour into gallon electric or hand-crank freezer. Churn and freeze according to manufacturer's directions until firm, then carefully remove paddle. Seal ice cream container well. Fill ice cream freezer to top with crushed ice and ice cream salt. Cover freezer tightly and set aside several hours to let ice cream season. Or spoon ice cream into another container which has been chilled and store in home freezer to season. Makes about 1 gallon