This page explains how to start and complete the Cooking with Koko side quest.
How to Start the Quest
Before you can start this quest, you must first complete the quest Koko's Kitchen. Then you can talk to Koko outside of the produce store in Kakariko Village between noon and 7PM to start the quest.
How to Complete the Quest
Give Koko a stick of Goat Butter to complete the quest. You can purchase them from the produce store that Koko is standing outside of. You will get a Hot Buttered Apple, and the recipe can be found in the record of the Cooking With Koko quest in the Adventure Log.