This Quick Cake can be made in quickly in a blender and there is a hidden surprise within it, watercress. The addition of watercress adds some leafy green protein and also makes the inside of the cake a lovely light green color, ideal for St. Patrick's Day. This simple cake makes a great base for you to experiment with icings, frostings, or other garnishes. To keep it classy, sprinkle powdered sugar on the finished cake after it has cooled.