Like anyone else who was asked to make or bring brownies to a function @ the last minute, you're looking for a quick fix. It's here & has continued for over 30 years & there's a lot of latitude for how you use ingredients in your pantry.
Combine softened butter with melted chocolate in food processor adding sugar, process until creamy (you may use stand or hand mixer also.). Add eggs one at a time. Pulse processing after each egg.
Add flour with vanilla till just mixed. Add marshmallows, nuts and 1 cup chocolate chips just until mixed. I usually do this by hand so these ingredients don't get pulverized.
Pour into 9 X 13 inch , lightly greased (sprayed) pan and bake at 350 degrees for 30 min. When brownies separate from sides of pan and has crust on top, remove from oven and spread the remaining cup of choc chips evenly over top. Give them approx. 5-10 to melt so you can spread them evenly with an off set spatula or the back of a spoon. This is great with ice cream or plain when cool.
Please feel free to use a brownie mix (like the SAM's club variety or Walmart brand). Follow the instructions for the brownie on the box you will still need to add chips, nuts, and marshmallows.