Ah, I had a thought, given your update.
What is the likelihood that, really, MIL likes her own shortbread, and only her own shortbread, for her daily "treat," but feels it would be rude to say to you "I would prefer to have my own cookies while we are visiting so I'll bring them along." ? The "I made them for DD!" would be a perfect cover - except for the fact that DD is now noticing.
If you think this might be the case, then I'd try the route of "You and DD make them here together", as that would address each part of the problem. There would be a full batch, sod DD would have more cookies, and MIL would have her daily snack.