- 8 tablespoons (1 stick) unsalted butter, plus more for pan
- 1/2 cup unsweetened cocoa powder
- 1 cup packed light-brown sugar
- 1 large egg, lightly beaten
- 1/2 cup all-purpose flour, (spooned and leveled)
- 1/2 cup milk
- 1 teaspoon vanilla extract
- 1 cup mini marshmallows
- 1 cup coarsely chopped walnuts
- 1 cup semisweet chocolate chips
- Preheat oven to 350 degrees. Lightly butter an 8-inch square pan. Line with parchment paper, leaving an overhang on two sides; butter the paper. Set aside.
- In a medium saucepan, place butter and cocoa; heat over low, stirring constantly, until butter has melted and mixture is smooth. Remove from heat; stir in sugar, egg, flour, milk, and vanilla until combined.
- Spread mixture evenly in prepared pan. Bake until a toothpick inserted in center comes out clean, 25 to 30 minutes. Remove pan from oven; sprinkle top evenly with marshmallows, walnuts, and chocolate chips. Return to oven, and bake until marshmallows puff, about 3 minutes. Cool to room temperature on a wire rack, about 1 hour. Then refrigerate until topping hardens, about 1 hour. Use paper overhang to lift from pan; cut into 12 bars.
Store at room temperature in an airtight container up to 5 days.