I believe this question should have stayed on EL&U, it was actually a duplicate of this question whose answer, however, merely states that that which is formal, without providing a reference. And I don't blame the user, it is not easy to find such a reference.
I did manage to find a comment on this in the CaGEL, p. 1036. After providing two examples:
- [i] It would mean abandoning that which we hold most dear. [antecedent + clause]
- [ii] It would mean abandoning what we hold most dear. [fused relative]
It goes on to analyse them:
These are syntactically equivalent (though [i] belongs to very formal style). Syntactically, that in [i] is antecedent, with which we hold most dear an integrated relative clause modifying it, but in [ii] what corresponds to that and which combined, so that it is not possible to separately identify antecedent and relative clause - hence the term fused.
So there you go, this reputable source confirms that that which and what mean the same, but are syntactically different, and that which is very formal. As long as your text is formal, regardless of the field or type of writing, you can use it safely.