The other day I baked some Nutella cookies (the famous four ingredient recipe which calls for egg, Nutella, vanilla and flour), they tasted awesome but the next day they became hard and chewy. A little Google search and figured they must be stored in an airtight container in the fridge. I bake several types of cookies but I have problem storing them so that several unknown factors don't affect their taste and texture. How do you figure out where is the best place to store homemade cookies to preserve flavor? Does it depend on the ingredients, as in if there is milk, store in the fridge?
If the cookies are baked, I cool them completely. Then, I put them on a plate and under a cake dome. They look nice, don't get smashed or crumbled, keep their flavor, and maintain their texture for a few days.
I do not store baked cookies in the refrigerator because I have had problems with the cookies becoming dry (even in an airtight plastic bag).
If I don't need to bake all of the cookies at once, my best results come from freezing the cookie dough and baking on demand.
I mostly bake chocolate chip or sugar cookies. So, my answer is based on those types of cookies.
I worked in a professional bakery and also have baked massive amounts of cookies every year for Christmas for as far back as I can remember. My preferred method of storage is in airtight containers in the freezer and doling out however many cookies at a time as needed. I find that any baked goods that are frozen soon after baking will have the same characteristics as just-baked items. It's the only way that I can bake for a few weeks and give fresh cookies as gifts and send through the mail. There are a few varieties that I do need to layer between waxed paper sheets, like chewy oatmeal, but all the cookies I've ever made through the years come right back to life once defrosted.
The cookie jar came into being for a reason. It is a place to keep cookies in a controlled environment that is not so open as to dry the cookie out, but is not so air tight that moisture from the cookie can't escape the container.
Hard dry cookies (or at least harder and dryer than intended) are no good. And floppy crumbled cookies from sitting in a humid sealed container are no good either. Hence the cookie jar where cookies can be kept for weeks before completely drying out, but not losing their integrity due to being trapped in with their own moisture.