Yummy' Yums are chocolate candy bars from the purge planet from the episode "Look Who's Purging Now". They contained in the past a substance known as Purgenol, which can make those that consume it aggressive and violent.
In the episode "Look Who's Purging Now", Rick and Morty stop by a general store on the Purge planet to purchase windshield wiper fluid. While at the store, they are given two free Yummy' Yums by the shopkeeper to celebrate the upcoming purge festival. During the festival, Morty displays highly violent behavior and kills several of the inhabitants attacking him. When they are leaving the planet and Morty is talking about how he is ashamed of his actions, Rick tells him about the candy bars containing Purgenol and its effects, saying that "His character's protected" as a result. However, the camera then pans to one of the bars, revealing that they no longer contain Purgenol.