Hi Sarah,
I would typically look for similar sypmtoms to what your LO initially experienced. Unless there are additional, very obvious ones, like hives, kwim?
The thing with butter and baked goods is that sometimes a baby can tolerate a little bit (through BM) every now and then, but if it builds up too quickly, or the gut hasn't completely healed, then the symptoms could appear later. It could be that your baby's digestive system has healed and matured to the point that hidden dairy and soy can be tolerated (or even anything through your BM).
If you want to move forward with challenging I would keep it to what you already had.....the soy, baked in butter, and choc chips. Try a little every day and see how it goes, then move on to all out hidden dairy, and move forward to yogurt, cheese, etc. Watch for the same symptoms or anything really out of the ordinary. OR you could just go full force and go for it with all dairy soy. At 9months I would not, BUT I was VERY cautious as DD was so sensitive to everything!
My ped said I could try food challenging at 9mo but did not say what to look for (I don't think she really felt there was a big problem )
Very few really seem to think it's an issue! Which is mind blowing to me!
Good luck! Please keep us updated on how it goes. We'll walk with you through the challenges!!