Grease an 8 by 8-inch baking pan with nonstick cooking spray.
In a large saucepan over medium heat, melt the butter. Add the marshmallows, Nutella, and peanut butter and stir until the mixture is melted and creamy. Stir in the cocoa powder.
Place the cocoa crispy rice cereal in a large bowl. Pour the butter mixture over the cereal and stir until the cereal is well coated. Spread the mixture evenly into the prepared pan and refrigerate for 10 minutes, or until cooled and firm. Slice into squares.