Butter an 8-inch square baking pan. Line the baking pan with parchment paper letting the paper extend up the two short sides of the pan and overhang slightly on both ends. Butter paper; set pan aside.
In a large saucepan over low heat, melt butter. Add marshmallows and cocoa. Stir frequently until melted, about 6 minutes. Remove from heat and stir in rice cereal. Using a piece of wax paper, press rice mixture evenly into prepared pan; Sprinkle with crushed peanuts if desired and drizzle with melted chocolate. Let cool to room temperature; cut into 16 bars.