Cooking the modified custard in a double-boiler
Image by ttstam
This is a technique borrowed from Chinese cooking – the corn starch / liquid mixture is slowly stirred in to the vanilla/sugar/milk mix. Corn starch has amazing thickening power, but if added incorrectly it will cook into hard lumps.
Most of the time in Chinese cooking, the juices from the dish is heated up to boiling, and then corn starch and water mix is then drizzled in, while the cook stirs furiously with a spatula. It is sometimes referred to a "glass sauce" because of the translucent sheen the sauce attains. Anytime you go to a Chinese restaurant and there is a sauce with the dish, more than likely it was made with this technique. Using the leftover juices from the stir fry transfers the maximum flavor to the sauce.
Digression aside, we can’t exactly bring the milk to a boil (worse case, it will actually taste burnt, which is what happened the first time I tried making this – half-gallon of vanilla goodiness down the drain. *sob* ). So, instead we keep stirring and add the corn starch mix in and make sure that the corn starch is evenly distributed in the mix – then bring the temperature up until the starch starts its cross-linking. This usually occurs between 70-85 deg C (160-185F). A double boiler gives the even heat required for that