From the Super Mario Wiki
Cake Mix is an item found in the Paper Mario series that heals one FP. In the Shy Guy's Toy Box, an unlimited source of Cake Mix can be found being held by a few Shy Guys in the Shy Guy's Playground. Cake Mix can bought at the Pianta Parlor in Paper Mario: The Thousand-Year Door for six Pianta Tokens (or eighteen coins). In Super Paper Mario it can be purchased at the Itty Bits in Flipside for 25 coins or collected as an item drop from a Moon Cleft or Goomba.
In Paper Mario, Mario can take it to Tayce T. so she can cook it to make a Cake, which restores fifteen FP. If Mario takes Cake Mix to Zess T. in Paper Mario: The Thousand-Year Door she'll make Mousse Cake, which gives fifteen FP. When Cake Mix is cooked with other items, the resulting product can have helpful attributes.
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