Melt 3 tablespoons butter and a 10-ounce bag of marshmallows in a saucepan over low heat. Remove from the heat and stir in 6 cups crispy rice cereal. Press into a 9-by-13-inch baking pan. Spread with 12 ounces melted bittersweet chocolate and sprinkle with nonpareils, then let harden. Cut into small squares.