Preheat oven to 350. Line a 13x9 pan with parchment paper or foil allowing some to hang over two sides and spray with cooking spray.
Melt 1 cup chocolate chips and butter over a double boiler. You can also carefully melt them in a microwave safe bowl for 1 or 2 minute time intervals, stirring in between.
Remove from heat and stir in sugars and salt, then eggs and finally flour. Stir just until combined. Spread batter evenly in pan and bake for 30-35 minutes or until a toothpick insterted comes out clean.
Remove from oven, sprinkle with remaining chocolate chips, then marshmallows and nuts. Bake for about another 5 minutes until chocolate chips are shiny and marshmallows are puffed. Allow to cool and remove from pan.