I want to add the smell and flavor of banana in my cookies. I'm not sure whether I should substitute APP-flour or butter with banana. Thank you.
Banana adds structure & moisture to baked goods, so you'd actually have to replace some egg, some butter, and some flour. When doing complex substitutions like this, it's usually best to find a recipe that's actually for the kind of cookie you want, in this case banana. Otherwise, you could just add some banana to the recipe as it is and go from there.
Of course, an easier method would be to use banana extract, which would replace the vanilla or other flavoring in your cookie.
You can generally replace oil in muffins and cookies with mashed fruit (applesauce, bananas, etc.) I've had good luck with replacing up to 2/3 of the oil with an equal volume of mashed fruit in muffins without problems.
Butter however is a bit trickier, as it depends on how it's being used in the recipe. If it's melted, it shouldn't be a problem to substitute. If it's creamed, it's going to create a completely different texture.
I'd agree wth sourd'oh : there are plenty of banana cookies out there ... I'd start from one of those, and if you don't like how they come out (eg. too cakey or too chewy), then adjust the recipe for your tastes.