Can I substitute just yolks in a recipe that calls for a whole egg?
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.