In a strong blender or food processor, blend the cookies until they are a fine powder. Add the melted butter and blend until totally smooth. If necessary, add more butter 2 Tbsp at a time.
Store in the refrigerator, but microwave before eating to soften (it will harden upon cooling, but easily becomes spreadable again with heat).
Recipe by The Kitchen Paper at http://c02.ddd.mwp.accessdomain.com/nutter-butter-cookie-butter/