How can I rescue an undercooked pastry cream? Yesterday I attempted to make pastry cream (my goal was to use it as a filling for creme puffs). After taking it off of the heat and letting it cool to room temperature I put in the refrigerator. That evening I checked on it to find that it is still very thin and liquid, I imagine that I had not cooked the egg yolks long/hot enough. For reference the recipe I used is this one, I followed the steps supplied to the letter with the one exception that I must have not cooked for long enough after returning the pan to the heat once all the ingredients had been combined.

Is there some way to salvage my pastry cream?


perhaps it may be a viable option to simply return it to a sauce pan and cook (with whisking) until thickened
perhaps there is some other dish I could make with the yet-unthickened custard (one where it is cooked in the oven maybe?)


Any advice on the above bullets, or novel ideas, are greatly appreciated.
When this happens to me, I am distrustful of a simple reheat. It is possible that it was undercooked, but also possible that it was maybe overcooked/understirred and that most of the binding proteins solidified on the bottom and too little of them remained in the milk, or maybe that the ratios were simply wrong. 

Your specific recipe also contains starch, so if you truly undercooked, then it's very probable that the yolks dissolved your starch, so you are left without any starch binding power. 

The most secure way is to throw it out and make another batch, this time boiling so that it blubbs for at least a minute. If you really want to save it, just use another thickener. In pure custards, I tend to add starch if they don't turn out. If there is already starch in there, I prefer another thickener such as xanthan. First, I don't know why the starch didn't work the first time, but repeating it somehow feels wrong. Second, it can get too much of a starchy/floury taste. 

The "other dish" is not something that can be answered on a Stack Exchange site. You have limitless possibilities to do with it whatever you want, starting with drinking it as it is. Anything which you can make with a custard which was made thin on purpose can be made with your failed custard too.