Preheat the oven to 350° and grease a 10x10 cast iron skillet.
In a small saucepan, melt the butter. Transfer the butter to a large bowl and whisk in the sugars.
Add the cocoa powder in mix well, now add in the eggs and whisk until the batter is silly smooth. Add in the flour, baking powder and salt. Whisk until everything is combined and there are no lumps.
Line the bottom of the pan with graham grackers. They can be broken pieces, it doesn't need to be perfect!
Spread half the batter over the graham crackers. Tear the marshmallows in half and plate 16 halfs all over the batter. Add in the chocolate chunks or chips.
Take the rest of the batter and spread it over the marshmallows and chocolate. Top with the remaining marshmallows and take broken graham crackers and stick them in the batter so they are sticking out.
Bake the skillet for 35-45 minutes. It will seem slightly undercooked. The brownie sets as it cools. But it's best served warm, especially over ice cream!
Notes
To make this brownie gluten free, replace the flour with an all purpose gluten free flour. If you don't have a skillet, use a cake pan.