From the Super Mario Wiki
A Whacka Bump is an item in the Paper Mario series. It restores 25 HP and 25 FP. It can be sold for up to 100 coins. According to Whacka's Catch Card in Super Paper Mario, the Whacka Bump is actually a form of pastry.
They are obtained by whacking a Whacka on the head with a Hammer. After eight hits, Whacka disappears and the player cannot obtain any more Whacka Bumps.
Mario collects a Whacka Bump
Whackas and Whacka Bumps also appear in Paper Mario: The Thousand-Year Door on Keelhaul Key. Once again, eight hits make the Whacka disappear for the rest of the game. The only other way to get a Whacka's Bump is to cook a Mystery until the player is lucky enough to obtain one. Having Zess T. cook one will result in a Zess Special, while cooking it with a Golden Leaf results in a Zess Deluxe.
The Whacka Bump is also available in Super Paper Mario, in which it heals 30 HP & cures poison. At the arcade, it can be traded for 300 Flipside Tokens, or sold for 150 coins, making it one of the more profitable items in the game. Whacka can also be found in Chapter 5-1. In the first part of the level, Mario should go to the far right wall. He then should use Dottie to shrink and flip to 3D. Whacka resides inside the hole. Cooking one in this game yields a Roast Whacka Bump, which raises its selling price to 200 coins, but it only heals 5 HP, not 30.
Paper Mario: The Thousand-Year Door sale prices
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