IIRC, she never actually said that. Someone at the time wrote that her attitude was such that she would say something like that.
She didn’t understand that there was a shortage of all grains and thus the bread shortage. She thought it was just one kind of bread that was in short supply. So she suggested that people could eat a different kind of bread made from different grains. English propagandists translated that as “cake” or portrayed it as her being carelessly indifferent to the peasants’ suffering when she was not.