1/2 cup unsalted butter, cut into pieces
1 bag (12 ounces) semisweet chocolate chips, divided
1/2 cup granulated sugar
1/2 cup packed light-brown sugar
1/2 teaspoon salt
2 large eggs
3/4 cups all-purpose flour
1 cup mini marshmallows
1/2 cup chopped nuts, such as pecans or walnuts
Preheat oven to 350 degrees. Spray an 8-inch-square baking pan with vegetable spray. Line pan with foil, leaving an overhang on two sides, then spray the foil.
In a heatproof bowl set over 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, eggs, and flour, stirring just until combined. Spread batter evenly in prepared pan.
Bake until a toothpick inserted in centre 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 the foil overhang, lift cake onto a work surface; cut into 16 squares.