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Line the bottom and sides of an 8-inch-square baking pan with parchment paper or foil coated with cooking spray. (Leave enough of an overhang to be able to lift the brownies out of the pan later with the liner.) If you don’t plan to let the batter rest at the end of Step 3 before baking, position a rack in center of oven and preheat to 350°F.
Thoroughly whisk flour, baking powder and salt in a small bowl.
Melt butter in a medium saucepan until sizzling. Remove from the heat and stir in cocoa, then sugar. (The mixture will look like a mass of very dark brown sugar.) Add egg, egg whites and vanilla. Stir briskly until smooth and glossy. Stir in the flour mixture until just incorporated. Then stir briskly for about 50 strokes. Scrape the batter into the prepared pan and spread it evenly. The batter will be very shallow in the pan. If you have time, cover the pan and refrigerate for a few hours or up to 12 hours. (This hydrates the cocoa powder and flour and brings all the flavors into sharper focus.) Remove from the refrigerator about 30 minutes before baking; preheat oven to 350°F.
Quarter marshmallows with an oiled knife. Distribute the pieces over the batter, pressing them in. Distribute nuts and chocolate (or chips) around the marshmallows.
Bake the brownies until the marshmallows are golden brown, 25 to 35 minutes. (If you can find a place to test for doneness where there is no melted chocolate or marshmallow, a toothpick should come out with some moist, gooey batter clinging to it. Brownies in a metal pan will bake faster than those in a glass pan; if you’re unsure, bake a few minutes longer to make sure the brownies are done in the middle.) Let cool completely in the pan on a wire rack for about 2 hours. Lift the ends of the pan liner and transfer to a cutting board. Cut into 16 brownies.