Whisk icing sugar and water until blended; spread onto top of Napoleon. Melt remaining chocolate; pour into resealable plastic bag. Cut small piece off one bottom corner of bag; use to pipe chocolate into 3 lengthwise lines on top of Napoleon. Drag toothpick crosswise over chocolate lines. Wipe toothpick clean, then drag, in opposite direction, across chocolate lines next to first swirled lines. Repeat until chocolate is evenly swirled on top of Napoleon to resemble photo. Refrigerate 30 min. or until chocolate is firm.