Cook very slowly, stirring almost constantly until desired thickness (when you put a little in a spoon and cool a little and it seems to be thickening - remembering it will thicken more after refrigerated. On a candy termometer it should register about 185 degrees. Remove from burner and to add vanilla - put a little custard in small bowl and add vanilla to it gradually while stirring. Then add that mix to large container of custard gradually, stirring well. Let cool and ten refrigerate until well chilled. Store in glass jars with lids.