Line an 8x8 or 9x9 square pan with foil, leaving an overhang to make "handles" and spray with non-stick cooking spray. Set aside.

Place Oreo cookies in a food processor or blender and pulse until ground into crumbs. You can leave some bigger pieces for texture.

Place marshmallows and butter in a large microwave safe bowl. Heat for 1 1/2 to 2 minutes until puffy.

Add cookie crumbs and stir quickly to combine.

Place into pan and press to even out.

Let set 10 minutes. Remove bars from the pan with handles of foil; cut and serve.