Halfling favored class ability: "Increase the capacity of the kineticist’s internal buffer by 1/6 point."
and
"Healing Buffer (Su)
At 6th level, a kinetic chirurgeon’s internal buffer has double the usual maximum size, and she can use it only when she would accept points of burn for the kinetic healer wild talent.
This ability alters internal buffer."
Does the bonus from the halfling favored class ability get doubled by Healing Buffer?
If no, would the internal buffer gained from favored class still be limited to healing only?
Or can you just not take that favored class ability?
The bonus would still be limited to healing, since the internal buffer that is getting modified was altered. Internal buffer only got altered, so you should still be able to take the FCB for it. But whether it counts double or not is up to interpretation. My feeling is that there would be no doubling, but I wouldn’t be surprised at a GM going the other way on that one.