I always use heavy cream with enough Jello instant pudding sugar free mix to make it thick enough for decorating...add vanilla and other flavorings to it and it's not too bad...certainly not less fattening than the real stuff though!
Good cake, the Sweet N Low brand chocolate cake mix but use sugar free Dr. Pepper or Sugar Free chocolate soda (hard to find) instead of the water in the recipe...pretty good for sugar free cake. All the scratch recipes reminded me of just sweet pancakes (be-yuck-O)