I don't agree that you should only cook the pastry cream for a few seconds once it comes to the boil. It's important to cook it for a couple of minutes to ensure the amylase in the egg yolk is deactivated to prevent it digesting the amylose in the starch. (You don't have to worry about the mixture curdling since the cornstarch or flour is protective at this higher temperature.) The extended cooking also aids the gelatinization of the starch molecules for increased stability. Failure to do this can result in a cream that thins over time - not a problem if you intend to use it quickly, but an issue for more than a day or so. The chilled cream sets quite firm, but a good beating soon loosens it up to the correct consistency.

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