The Chuckola Reserve is a legendary soda located deep in Chucklehuck Woods; it is a form of Chuckola Cola. During the events of Mario & Luigi: Superstar Saga, Mario and Luigi had to find the Chuckola Reserve to cure Queen Bean, who had swallowed a Belly Blech Worm. (Such an incident morphed her into a hideous monster; only the Chuckola Reserve could change her back.)
They eventually find the soda inside Mother Chuckalola, along with Bubbles, the founder of Château de Chucklehuck. Bubbles had worked on the soda for one thousand years, telling it joke after joke so it would mature. However, Bubbles was so protective of his Chuckola Reserve that he had it transformed into a monster, the Chuckolator, whom the Mario Bros. had to fight.
After the Chuckolator was defeated, Bubbles sent the soda and the Mario Bros. underground. Mario landed in the barrel containing the soda, filling him up with it. Luigi surfed the barrel back to Beanbean Castle and used the cola to heal the queen.