My MIL clearly read the same handbook as those of CurlyHairedAssassin and tea towel.
Every year I bake cakes for DP, DD & DS; and every year MIL stops off on the 1hr+ drive to buy one. I used to get really miffed as I took it as a personal slight and, like others, find it incredibly wasteful; now I have resigned myself to the fact that she thinks she is being helpful/generous/kind rather than devious/sneaky/malicious and don't say anything.
That being said, I was a bit
one time when she said to P that she had stopped into the supermarket to buy it as "I didn't know if Jammy would have remembered bothered "
BTW, if for whatever reason I hadn't made one, we have 3 supermarkets in a 1mile radius....
HOWEVER, just because I have made my peace with it doesn't mean I think You should - there is far more going on here than a MIL trying to be helpful.
I think you need to clearly state what is happening and what the consequences will be if she chooses to defy you.
I am baking DS's cake.
You ARE NOT to buy one. I would be a waste of time & money as it will not be used.
You are however welcome to help out in any of these other ways.... List x,y, z
Be firm and if she does continue to try and guilt trip you then it really is time for you to calmly state a few home-truths re: your (so far) openness / desire in keeping her involved in the life of her DGS. Ultimately, it is not your responsibility to keep that relationship going, it should be his father. Maybe it is time to point out you are extending a courtesy that many others in your position would not.
Oh, and the cake you have planned looks amazing and really fun, I bet he loves it!!