I am not kidding.
My dad, who attended a french boarding school in the 60s, insists he was taught that she did this. He refuses to believe any Google results that I show him. I have tried to explain that he must have misunderstood the phrase "Let them eat cake" and what it actually implies. I have tried to explain that she probably never said it anyway.
He is insisting that she gave an order to deliberately poison cake to give to her starving subjects to kill them off and the outrage of this event is what sparked the French Revolution.
He is a Boy Scout merit badge counselor and is actively teaching young boys this nonsense.
I am at my wits end.
I am asking if anyone would be interested in a Zoom call to set him straight. Anything anyone thinks would help would be appreciated.
I cannot answer your main question, but if you want a resource for talking with your father about this, you may want to check out the primary sources I referenced in this answer to the question of whether she actually said it.