I would say something along the lines of: "What can I say? I'm an ass guy! Your *insert ONE feature about her face here* is/are kinda sexy too though..."
This way, you're not giving her the FULL compliment shes looking for "oh your face is so pretty too!", but rather letting her know that you KNOW what you like, and that isnt going to change cause she's decided to fish for a compliment out of you.
The other bonus of this response is that you focus in on ONE feature of her face (her eyes for example, or her dimples, or lips..or w/e), so it seems a little more SINCERE and GENUINE than saying "oh ya, your face is smokin hot too!" You maintain that you love her ass, you give her a sincere compliment about a feature of her face, which will make her absolutely melt!
Cheers!