- 1/2 cup (1 stick) unsalted butter, cut into pieces, plus more for pan
- 1 bag (12 ounces) semisweet chocolate chips
- 1/2 cup granulated sugar
- 1/2 cup packed light-brown sugar
- 1/2 teaspoon salt
- 2 large eggs
- 3/4 cup all-purpose flour, (spooned and leveled)
- 1 cup miniature marshmallows
- 1/2 cup chopped nuts, such as cashews, pecans, or walnuts
- 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.
- 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.
To prevent toppings from sinking into the batter, sprinkle them on after brownies are baked. An extra 5 minutes in the oven melts the topping together for easy slicing.