I would use a good quality chocolate sandwich cookie (like Oreo). Split them, scrape off as much filling as you can, then grind them into crumbs.
I have ground the whole cookies - including the filling - into crumbs to use for a crumb pie crust in place of chocolate wafers. That would work for you too, except that the filling adds extra sugar that might make your cookies too sweet. It would just be easier to remove the filling before grinding the cookies.