During Substory #62: Grannys Request Part I you will be asked to collect Handmade Cookies. This guide will tell you Where To Find Handmade Cookies In Yakuza Kiwami 2 so you can return to Granny White with Handmade Cookies to complete the Substory.
Once you have unlocked the Substory #62: Grannys Request Part I, Granny White will ask you if you could find some Handmade Cookies. These are a very specific item, one which you can only purchase in a single store.
In order to complete the Substory you need to return to Kamurocho. Once you are back in town head to Poppo (West Shichifuku Street). Inside the store, speak with the clerk and he will open the shopping menu.
From here you can navigate down and purchase yourself some Homemade Cookies.