Sugar free yellow cake mix (made with Splenda and reduced in calories by about 50 calories a slice when compared to the original cake mix), diet Crush, and lite frozen whipped topping. Measure out 12 ounces of diet soda. Pour the soda into the cake mix. Whisk for 30 seconds. Bake according to the directions on the box. Let the cake cool (if you have the patience for that sort of thing). Top with thawed whip cream.