Heat oven to 350 degrees F. Line an 8 x 8-inch baking pan with nonstick foil, overhanging ends of pan.
In medium-size bowl, combine cocoa powder, flour, baking powder and salt. Set aside.
In a large bowl, whisk together oil, sugars, egg whites and vanilla. Stir in cocoa mixture. Spoon batter evenly into prepared pan. Sprinkle walnuts over batter. Bake at 350 degrees F. for 25 minutes. Scatter the chips and marshmallows over the top and bake for an additional 5 minutes. Remove from oven and cool completely.
To slice, coat a sharp chef's knife with cooking spray and cut into 25 squares, using a rocking motion. Re-spray knife frequently.