In a heavy saucepan, combine the first five ingredients. Cook over medium-low heat, stirring constantly, until the mixture is thick enough to coat a metal spoon and reaches at least 160°. Remove from the heat; cool quickly by setting pan in ice and stirring the mixture. , Cover and refrigerate overnight or freeze immediately. When ready to freeze, pour custard and vanilla into the cylinder of an ice cream freezer. Freeze according to the manufacturer's directions.