Pour batter into greased 9″ x 13″ baking pan and smooth into an even layer.
Layer Oreos in a single layer over brownie batter, in four rows and six columns. Reserve remaining cookies for later.
Bake for at 350 degrees for 18-20 minutes or until just set.
Add package of Oreos to bowl of food processor.
Pulse cookies in food processor until they are coarse crumbs.
Add marshmallows and butter to large saucepan.
Stir marshmallows and butter together over low heat until melted and completely combined.
Add Oreo crumbs to saucepan.
Stir to combine.
Remove brownies from oven.
Press marshmallow mixture gently into an even layer over top of warm brownies.
Crumble remaining Oreos over marshmallow treat mixture, gently pressing them in to adhere. Allow to cool completely before cutting.
Notes
TIP: I like to use the same saucepan that I used to make the brownies. Less dishes to wash and I figure nobody is going to complain about a little extra chocolate in the recipe!