I have a cookie recipe that only needs one whole egg. Since I have lots of yolks from other baking, can I use 1 yolk with something else (like a Tbsp. of water) or can I use 2 egg yolks?
From Cook's Thesaurus Eggs:
Substitute 2 egg yolks for each whole egg. This is higher in fat, but works wonders in sauces, custards, and cream fillings.
I suggest you use two egg yolks, and perhaps slightly, almost unnoticeably, lower the amount of fat you put in the dough.
It depends on what your goal is. It will work chemically, per the other answer. But you will definitely get a much stronger egg flavor in the dish. In some dishes, this is acceptable or even desirable. In others, it could be distracting.
My preference is to use the yolks in recipes that actually call for extra yolks. For example, ice cream. :)