They are Krabby Patties with their buns smothered with grease.
Role in episode
They are created by SpongeBob after he drops them into a puddle of grease by accident. Even though they are not healthy, the Krusty Krab sells them until the health inspector comes to see the mess. They are created by soaking Krabby Patty buns in grease for one hour. The new menu item became a hit until Plankton invented Crispity Crunchities to win back the hungry crowd.
This bun type does not appear on Galley Grub, but rather on a sign outside the Krusty Krab.
The name "Yummy Bunz" was also adopted by Viz Media for the English version of the Japanese anime Doraemon, which airs on Nickelodeon's rival Disney XD. It was used to describe a Japanese confection called "Dorayaki," which became Doraemon, the main character's, favorite. However, it spelled as "Yummy Buns," as the name "Buns" is spelled with an "s" instead of a "z."