How can I rescue an undercooked pastry cream?
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.