I'm a licensed foster home so we have a lot of kids that come into our home with neglect and emotional issues. More often than not, they have hoarding and stealing issues with food. We keep everything locked in a pantry except that which is okay for them to eat at night....fruit, veggies, peanut butter crackers, etc. Yeah it is a pain, but worth my sanity and grocery bill!
I also use child locks on our cabinets so our son can't devour the pantry. He loves his treats!
We've also used a snack box that they can keep in their room, when it's gone for the week it's gone...this depends on functioning level and how successful it is.
There was a point that locking everything wasn't working so we installed a motion detector to alert us when the kids were in the kitchen.

I really don't even call anything "stealing" with asd kiddos. I agree that it's impulse control. Mine I know cannot help it.

I've found that most times it is not the act of stealing that it is about. Usually it's an emotional response to something. With our son it's also about the emotional baggage from times before he was with us and also about the lack of impulse control. He just sees it and wants it right then. We leave a snack on the counter for him for first thing in the morning, so he doesn't try to commando the refrigerator and pantry open!

My oldest steals food. It seems to come and go in waves. We haven't found a way to stop him so at times we are a treat free zone.

Can treats be a reward for good behavior? If she has a sticker chart so she can see what she has to do to get the treat and when she will get it, it might decrease the stealing.