Preheat oven to 325 degrees. Line an 8×8 inch or 9×9 inch baking dish with parchment paper, leaving about a 1 to 2 inch over hang to make for easy transfer. Set baking dish aside.
In a small bowl combine flour, cocoa powder, and salt. Sift dry ingredients together or use a whisk to mix and break up any clumps in the flour and cocoa powder. Dry ingredients should be thoroughly mixed together and free of clumps.
In a large, heat safe bowl, melt chocolate and butter together by a double broiler or heating in the microwave, stirring in between each heat session. Chocolate and butter should be completely melted and smooth.
Add in sugar and mix thoroughly. Batter will be slightly gritty. Add in eggs, one at a time thoroughly mixing before adding in the next one. Add in vanilla extract and mix.
Slowly add in dry ingredients, folding them into the batter. You don’t want to mix vigorously because this may cause your brownies to be tough when they bake, just fold the batter until all the dry ingredients are incorporated. Brownie batter will be thick.
Pour about half the brownie batter into the bottom of your baking dish. Evenly smooth it out so it cover the bottom of the dish and creates a thin layer. Next, place oreo cookies over the brownie batter, placing them side by side to cover the brownie layer. You can also break the cookies in half to fill in gaps or spaces. Spoon some marshmallow topping over the cracks/gaps between the oreos and the batter. Drizzle topping over the oreos as well. Reserve some topping for the top layer.
Pour the rest of the brownie batter over the Oreo cookie and marshmallow layer. It may seem like there’s not enough batter to cover it but just smooth it out until it covers everything. Some oreos or marshmallow topping may peek through, this is fine. Spoon marshmallow topping in thick vertical or horiztonal lines over the brownie batter. Using a butter knife or toothpick, swirl the marshmallow around in different directions to create a marbled look. Add a few more oreos to the top layer, some whole, some broken.
Bake in the oven for about 40 minutes. Brownies are down when a tester inserted into the center comes out clean. Since there are cookies in the center of these brownies you may have some cookie on your tester.
Allow brownies to cool completely before serving, about 30 minutes or so. When ready to serve lift up sides of the parchment paper and lay brownies on a clean, flat surface. Using a sharp kitchen knife cut brownies into squares and serve. Any leftovers should be stored in an airtight container.