Preheat oven to temperature on back of brownie box. Line an 8" x 8" baking dish with a piece of crumpled parchment paper. Lightly spray parchment with non-stick cooking spray.
In a large mixing bowl whisk together brownie mix and the 3 ingredients listed on box of brownie mix (oil, water, egg).
Whisk in the hot cocoa powder until smooth and combined.
Spread 1/2 of the brownie batter onto the bottom of the prepared baking dish.
Press 16 Oreos into the batter to create one even layer. Spread remaining brownie batter on top of the Oreos and filling in the gaps. Smooth the t op.
In preheated oven bake per the directions on brownie box until edges are set. Center will be slightly fudgy. Let brownies cool for 20 minutes.
Melt Ghirardelli Milk Melting Wafers per directions on package. Pour melted chocolate on top of warm brownies still in baking dish and smooth evenly.
Sprinkle marshmallow bits across the top and slightly press into brownies using back of spoon.
Chill for 15-30 minutes in refrigerator to set up. Remove from baking dish, slice, and enjoy.