Place cookies in food processor & pulse until they are coarse crumbles
Line an 8x8 inch pan with parchment & give a light spritz with cooking spray
In a large bowl - combine marshmallows & butter
Microwave marshmallows on full power for about 1-1/2 minutes. They should puff up quite a bit.
Remove & immediately mix in the Oreo crumbles until fully mixed in
Transfer to prepared pan & press into an even layer
Top with remaining whole cookies & press in
Allow to cool completely before cutting.
Store in refrigerator in airtight container but bring to room temperature before serving