Jean-Jacques Rousseau
Confessions, book 6 (French: "Qu'ils mangent de la brioche"), Written c. 1766, published 1782
The phrase tells a story about a royal who does not understand poverty. Rousseau included it in his memoirs as an anecdote, not as a quote.
the myth (Marie Antoinette): Rousseau wrote the line about "a great princess" and never named her. He wrote it when Marie Antoinette was a child in Vienna, so the story cannot be about her.
Source: Wikiquote