Preheat oven to 350°F and grease an 8 or 10-inch cast iron skillet with butter.
In a medium bowl, combine cocoa powder, brown sugar, flour, salt, and chocolate chips.
Add the eggs and vanilla and mix well, slowly following with the melted butter.
Pour the brownie batter into the skillet, spreading out evenly with a rubber spatula.
Bake for 15 to 20 minutes until the brownie is slightly firm on top.
Remove from oven and sprinkle the chopped chocolate over the top of the brownie, then place marshmallows cut side down on the chocolate.
Broil until marshmallows are toasted as desired, keep a close eye on them so they don't burn, coloring will happen fast.
Remove from oven and serve warm with graham crackers.