6 tbsp unsalted butter, plus more for pan
1 bag (10.5 oz) mini marshmallows
1/4 c. unsweetened Dutch-process cocoa
6 c. crisp rice cereal
2 oz bittersweet chocolate, melted
Butter an 8-inch square baking pan. Line bottom and two sides with parchment paper, leaving a 2-inch overhang on both sides. Butter paper and set pan aside.
In a large saucepan, combine butter, marshmallows and cocoa. Cook over medium, stirring frequently, until melted, about 6 minutes. Stir in rice cereal. Press rice mixture into prepared pan; drizzle with melted chocolate. Let cool to room temp; cut into 16 bars.
To store, keep in an airtight container at room temperature, up to 5 days.