Preheat the oven to 350 degrees.

In a stand mixer or in a mixing bowl, cream together sugar, butter, and cocoa powder. Add the eggs one at a time. Then add vanilla extract and flour, scraping the bowl throughout the mixing process.

Grease a 9×13 pan with cooking spray. Pour prepared brownie batter into the pan and spread out evenly. Bake for 30 minutes.

Remove the pan from the oven and immediately sprinkle the marshmallows evenly over the baked brownies. Then sprinkle the graham cracker and chocolate bar pieces over the marshmallows. Place in the oven for two more minutes to allow the marshmallows to melt together.

Place in the fridge for one hour before serving, or on the counter top till the marshmallows have cooled. (If you cut into them too early you’ll have a delicious but incredibly sticky mess and they’re a little difficult to cut.)