The phrase "Let them eat cake" is famously attributed to Marie Antoinette, Queen of France, during the French Revolution. It is a misattribution and likely never actually said. However, the phrase is used to criticize the wealthy and powerful who are out of touch with the struggles of the common people.
If you're looking for a similar quote in *Beowulf*, it might be helpful to consider themes of feasting, wealth, and the social hierarchy within the story.