Cocoa Trees are naturally generated in new chunks, specifically Swamps and Jungles.
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Cocoa Trees

Cocoa Beans will naturally drop over time from the leaves, with a maximum of 8 under each tree at once.

Breaking the leaves may drop cocoa beans or cocoa bean saplings.

To plant Cocoa Bean trees, you must plant a cocoa bean sapling. Should breaking the leaves not work, a Cocoa Bean sapling can also be obtained by crafting a cocoa bean and a regular sapling together.