I think a professional trainer is the way to go as you've been having this problem for quite a while now.
I do want to warn about two methods:
The hand/claw on the neck can be very effective when done correctly, but when you touch a dog that is already escalating to zone 10, odds are it will turn and bite your hand. So timing is critical.
The second method is putting a dog on it's side or back and forcing it to stay there. This is also called the 'Alpha Roll' .... from an earlier post:
The 'holding down' is a form of the 'Alpha Roll' and should never be done to puppies or young dogs. It was developed by professional trainers to be used by professional trainers on severely aggressive dogs, after all alternative methods of training have been tried and failed. The 'Alpha Roll' would then be tried, only as a last attempt to save the dog from being PTS.
When it is used on younger dogs and puppies, it can have adverse effects and actually make them worse aggressors, or the opposite can occur and their spirits can be broken, and they end up being fearful, cowering, hiding under the bed messes, which then need to be retrained to begin trusting humans again.
I will say some members have gotten 'lucky' using this method, and I just want to say here that it was intended to be used only once. Repeated use will not improve matters and may cause the adverse reactions mentioned above, or worse.
From:
Alpha roll - Wikipedia, the free encyclopedia
Monks of New Skete, The (2002).
How To Be Your Dog's Best Friend. Little, Brown & Company.
ISBN 0-316-61000-3. "In the original edition of this book, we recommended a technique we termed "the alpha-wolf rollover"...
We no longer recommend this technique and strongly discourage its use to our clients.... The conditions in which it might be used effectively are simply too risky and demanding for the average dog owner; there are other ways of dealing with problem behavior that are much safer and, in the long run, just as effective."
What will stop her is to put a leash on her and sit with both dogs on the floor and wait for her to give the signal that she's going to attack. If you miss the signal, stop her from touching Buster by stopping her with the leash. Get her back to you and tell her to sit and put your body between her and Buster, blocking her sight of him. Walk towards her so she has to move back. This body language makes her see you as the boss. When she calms down, sit down and wait for the next time.
Do not provide triggers to make her react. No petting of both dogs or just Buster... this is teasing. Let it happen naturally. Once you stop her physically, the mental part will kick in and she'll begin to think before she does it again. Work on getting the timing just right, give a very firm 'Ah, Ah' when you see her thinking about Buster. Sometimes that alone will stop her... I've also seen it provoke an attack. If it provokes an attack, do the verbal correction a few seconds sooner the next time.
Stop giving treats for her to hide. Peek a Boo does this also and hides treats in his bed in the bedroom. He will challenge the other 3 for their treats until he collects them all, then he will attack anyone that enters the bedroom. He stopped doing this when I stopped giving him treats. Now I give him Cheerios, which he'll eat right away.
This is a process, meaning it will take time. But it will work. Peek was worse at night in bed. I slept with a leash on him, but still couldn't totally stop him. Now, sadly, he does not sleep in my bed at night. But this is what works for him, and keeps everyone else safe.
Once you effectively stop her attacks a few times, you should see a change in her behavior... attacks will be fewer and futher apart. Eventually, when the verbal interupt stops her, you can work with her without the leash, but keep it handy, you may need to put it back on for a bit. Yorkies are smart and will try you.