I made a vanilla custard and stopped stirring at times. How do I remove the burnt cornstarch from my custard?
The best case here is that, once you notice that it has burned on, you stop stirring. Then you let the custard cook up as gently as possible and remove it from the heat. Then carefully ladle out the custard, down to maybe a centimeter or two above the bottom, without disturbing the burnt part. Make sure to stay well above the burned part, instead of trying to push close to the "boundary" (which is in fact a gradual zone) between good custard and burnt layer.
If you have already stirred the custard, then it is still usually possible to save it. The good news is that people usually notice the problem after the starch has created a roasted-on solid layer on the bottom, but before it has turned to char. In this case, you continue cooking your custard as usual, then, as soon as it is ready, pass it through a sieve. The mesh fineness is the one appropriate for Turkish coffee or for sieving flour - not large holes like a colander or a salad sieve, but also not as fine as a chinois or a coffee filter. You won't get the roasted aromas out of the custard, but as they are hopefully not the stink of charcoal, they are unlikely to be perceived as unpleasant by the eaters.
The cases in which you cannot save it are:
- If by "burnt" you mean "charred to the point where it smells unpleasantly like burned food" - the simple rule is that you cannot get smell out of food. For details, see the questions fyrepenguin linked in a comment.
- If the custard has already set.
- If this is a custard that relies mostly on eggs setting, with only a little bit of starch added, and burning the bottom means that you also overheated the eggs to the point where they either split or you got lumps of fried egg on the bottom.