Line the bottom of an 8 inch square baking pan with parchment paper, leaving an overhang on two sides, so that later on you can just pull the brownie cake by this. Butter the baking pan and the parchment 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, 25 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.