After 3 seconds of no barking, let her have the treat. The next time she barks, require her to be quiet for 5 seconds before she gets the treat. Each time she is told to be quiet and succeeds, she will be rewarded. But eventually she will make a mistake, if for no other reason, she may now bark at you impatiently waiting for her reward. If she barks even one little wooflet after you've given the command, scold her immediately.
Timing is everything.
No barking for the next 3 seconds, then 5 seconds, then 10 seconds and so on. Within a single training session, you can teach your dog to be quiet for up to 1 or 2 minutes. This is major progress, because whatever excited her to bark in the first place is history, and she is likely to be quiet until the next disturbance. Sometimes a splash of water in the face will do the trick. You must find something that will instantly make your dog be quiet.