6 tablespoons unsalted butter, plus more for pan
1 bag (10.5 ounce) mini marshmallows
1/4 cup unsweetened Dutch-process cocoa
6 cups crisp rice cereal
2 ounces bittersweet chocolate, melted
Butter an 8-inch square baking pan.
Line bottom and two sides with aluminum foil, leaving a 2-inch overhang on both sides.
Butter foil; set pan aside.
In a large saucepan, combine butter, marshmallows, and cocoa.
Cook over medium heat, 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 temperature; cut into 16 bars.
To store, keep in an airtight container at room temperature, up to 5 days.
Martha Stewart Everyday Food Magazine