From the Super Mario Wiki
The Golden Leaf is an item found in Paper Mario: The Thousand-Year Door in Creepy Steeple. On the northern porch (where two Swoopers and a red ! Block are found), look for an opening in the grass Mario can go through with Paper Mode. Move to the right past the ! Block and hit the gold tree with Mario's hammer. This item recovers 10 FP when Mario eats it. If the player take it to Zess T., she will make it into Zess Tea. It is also used in many combination recipes. This rare item is also required for completion of Chef Shimi's trouble, along with a Keel Mango and a Mystic Egg.
The Golden Leaf also appears in Super Paper Mario. This variant recovers 1 HP and can be obtained by defeating an Amazy Dayzee or using a map from Flamm. Flimm occasionally has a Golden Leaf for sale as well. Having Saffron cook a Golden Leaf for the player will yield a Gold Bar.
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