Finding the perfect brownie recipe can be a challenge. Some turn out too dry, some turn out to gooey and some (like these) turn out just right. These have the perfect density—firm yet chewy. I like my brownies super-chocolatey, so these have added cocoa powder to bump them up a notch.
Preheat the oven to 350 degrees F and spray an 8" square pan with cooking spray. Line the pan with two pieces of aluminum foil, then spray the foil. Be sure to leave enough foil over the sides so you can easily lift the brownies out when cooked.
In a metal bowl, over boiling water, melt the chocolate and butter together, stirring with a rubber spatula until melted and combined. Gently whisk in the cocoa powder, then set aside and allow to cool slightly.
Pour the mixture into the prepared pan and bake for about 30 minutes.
While the brownies are baking, combine the chocolate chips, marshmallows and walnuts. Sprinkle the mixture on top of the cooked brownies, then return to the oven for an additional 5 - 7 minutes, or until the marshmallows and chocolate have begun to melt.
Allow to cool in the pan for about 10 minutes before lifting out using the aluminum foil and finish cooling completely on a wire rack.