Apparently this isn't new news, especially around the Weight Watchers crowd, however, I just heard about it.
Take any box cake mix and mix it with one can of diet soda - nothing else. Bake according to directions on box.
I just tried it with Betty Crocker Super Moist Triple Chocolate Fudge and a can of Diet Dr. Pepper Cherry Vanilla. Worked great!
Yellow cake mix & diet orange or 7-Up
Any dark cake mix and Diet Coke
etc.
170 calories per serving -vs- 270 calories