My little Alice is a growler at times as well (and hey she's a tortie too XD). She only started it once we started trying these natural freeze-dried treats. Every time I would give the girls these treats, she'd take hers and slink off growling at the other girls. As harmless as it seemed at the time, I too didn't want it to escalate further in the future.
We learned the "Drop it" command. If she slunk off, growling with her treat, I would say "Alice, drop it." and she would stop, drop her treat, and look up at me. I would re-pick up her treat, and re-present it to her. If she could take it without growling, she got lots of extra praise, and cuddles afterwards. If not, then rinse and repeat.
She's learned that if she just accepts the treat like every other treat or meal, and trusts me not to let any treat-stealing occur (I'm looking at you Samantha! XD), that she can have her treat quicker and eat it in peace. It's pretty rare that she will growl now when getting these treats and they are now one of their staple treats.
I wish you the best of luck with Pixie! She's a beautiful little girl.
Whatever method that works with Pixie to curb this habit now, the only advice I can give you is keep at it...torties are stubborn little brats, and they can sense when yer frustrated and about to give in...they've got ya then.