Whisk both sugars into the bowl with brown butter.
Whisk in the egg and vanilla until well combined.
Add in the flour, baking powder, baking soda, and salt. Use a rubber spatula to fold the mixture together just until a few flour streaks remain.
Crush up the remaining graham crackers. Add the crushed graham crackers, chocolate, and dollops of marshmallow creme into the dough and fold the mixture together just until no flour streaks remain.
Pour half of the brownie batter into your prepared square pan, smoothing it into an even layer.
Use your hands to tear up pieces of cookie dough. Flatten them in your hands, then place them on top of the brownie batter, trying to form a flat, even layer of cookie dough on top of the brownies. Use approximately 3/4 of the cookie dough for this layer.
Pour the remaining brownie batter over the cookie layer, smoothing it out into an even layer.
Tear up the remaining pieces of cookie dough, scattering them on the top layer (It will not fully cover the brownies - this is what we want so that the top is marbled!)
Bake for 45-50 minutes, until the edges have set and the middle is only *slightly* wobbly. (You can also check to make sure the internal temperature of the brownies is 200°F / 93°C to see when they're done.)
Allow the brownies to set in the pan until they come to room temperature. Serve and enjoy!