Do you really "have" to bake cookies for someone? If so, maybe tell yourself that your friend is NOT your aunt and she would not criticize you.

I know that's not much help but I don't know what else to say since I don't know what PTSD is in that context.

If you're really wired about it, you could "come clean" about it with your friend and use it as an opportunity to see if there's understanding there.