- 1/2 cup (1 stick) unsalted butter, cut into pieces, plus more for greasing 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 eggs
- 3/4 cup all-purpose flour
- 1 cup miniature marshmallows
- 1 cup walnuts, chopped
- Preheat oven to 350 degrees F. Butter an 8x8" baking pan. Line bottom with parchment paper, leaving an overhang on two sides; butter paper.
- Melt butter and 1 cup of the chocolate chips together in a double boiler or in the microwave on the chocolate setting.
- Stir in sugars and salt, then eggs, then the flour, stirring just until combined. Stir in 1/2 cup walnuts. 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 tops with remaining chocolate chips, then marshmallows and nuts. Bake until chocolate is shiny and marshmallows are puffed, about 5 minutes.
- Cool completely in pan. Refrigerate 1/2-1 hour for easier cutting. Use the parchment overhang to lift the brownies onto a cutting board. Slice into 12 squares with a large knife, wiping clean after each cut.