Preheat oven to 350 degrees. Grease and/or line pan with parchment paper.
In a small bowl combine flour, salt, and baking powder; set aside.
Combine melted butter, sugar, and vanilla in a large mixing bowl or stand mixer.
Add eggs one at a time and beat until well combined.
Mix in cocoa powder then slowly add in flour mixture.
Stir in 1/2 cup of nuts and 1 cup of chocolate chunks into batter. Pour into greased pan and bake for 30-45 minutes depending on size of pan.
In the last 5 minutes of baking sprinkle chocolate chips, marshmallows, and nuts on top. Bake for 5 minutes or until marshmallows are slightly puffed and toasted.
Allow brownies to slightly cool before cutting.