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Preheat oven to 350 degrees Fahrenheit. Butter an 8-inch-square baking pan. Line bottom with parchment paper, leaving an overhang on two sides; butter paper.
In a heatproof bowl set over (not in) a saucepan of simmering water, combine butter and 1 cup chocolate chips. Heat, stirring occasionally, just until melted, 3 to 5 minutes.
Remove mixture from heat; stir in sugars and salt, then eggs and finally flour, stirring just until combined. Spread batter evenly in prepared pan. Bake until a toothpick inserted in center comes out with moist crumbs attached, 30 to 35 minutes.
Remove from oven and sprinkle with remaining chocolate chips, then marshmallows and nuts. Bake until chocolate is shiny and marshmallows are puffed, about 5 minutes. Cool completely in pan. Using paper overhang, lift cake onto a work surface; cut into 16 squares.