The simple solution would be to eat more but that would result in body weight gain. Usually a gaunt looking face indicates one or a combination of three things:
1. Under Nourished (not eating enough)
2. Weak Facial Muscles
I'm going to rule out number 1. as you imply that your happy with your body. Numbers 2. and 3. are basically the same, as anyone can have weak facial muscles and the muscles in the face often become weak with old age.
To strengthen the weakened muscles you will need to do progressive neck and face exercises. When performing facial exercises, you are tensing the muscles and working them "against" pressure. The muscle becomes toned, which means it's either bigger because you're building muscle fiber and/or as the muscle tones it becomes dense.