How to add banana flavor to my cookies?
Banana adds structure &amp; 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.