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Preheat oven to 350 degrees. Butter an 8-inch-square baking pan. Line bottom with parchment paper, leaving an overhang on two sides; butter paper.
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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.
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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.
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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.