Iwagakure Cookbook (いわつくし料理帖 Iwagakure Ryōri jō?) is a manga-omake included in Volume 12.
Part 1[]
Yui presents Gabimaru with signs she made for the purpose of improving the efficiency of their test-tasting sessions and to help make better chestnut rice. After trying the first chestnut rice dish, Yui grades it with the signs for "yummy" and "sweet". She then proceeds to grade the second dish with the signs for "yummy" and "not sweet". While Yui holds up the "not sweet" sign for a third dish she grades, Gabimaru grades his dish by holding a sign up in the middle.
Part 2[]
Kumokiri lends his assistance in cooking and uses kikatsugan on how practicality is the goal to making ninja food instead of flavor. However, his point doesn't get across to Gabimaru, Shija, Kinkakubō, and Ginkakubō as they give the kikatsugan a low rating based off of taste, much to Kumokiri’s displeasure.
Part 3[]
Shija lends their assistance in cooking but is noticed by Gabimaru to have put something suspicious in their dish that makes it seem toxic. Shija assures him that there is nothing wrong and insists that he eat what they have served.
Part 4[]
Having found their assistance to be worthless, Gabimaru decides to make chestnut rice on his own using his own methods, resulting in a poorly made dish. While Kinkakubō and Ginkakubō refuse to eat it and Kumokiri gives his disapproval by holding up a sign for "yuck", Shija gives their approval after eating the dish in delight.
Part 5[]
After she tries Gabimaru's chestnut rice, Yui gives her disapproval by holding up the signs "not sweet" and "not yummy".