Preheat oven to 350 degrees and position a rack in the lower third of the oven. Spray a 9x9 glass pan with non-stick pan.
Combine flour, sugars, cocoa, and salt in a large bowl and mix with a whisk.
In a small bowl, combine water, applesauce, vinegar, and vanilla. Pour liquid over flour mixture and mix with the whisk until just smooth.
Add half the chocolate chips and stir to incorporate.
Scrape batter into prepared pan and smooth evenly. Sprinkle marshmallows, chocolate chips and walnuts and gently press into mixture leaving tips exposed.
Bake for 45 minutes until knife comes out with bits of gooey crumbles. Let brownies cool for 10 minutes and slice into 12 brownies.