Does anyone have a recipe for a chocolate cake made with sugar substitute ?

Try checking the manufacturer's web site(s). Each kind may have specific differences in its chemistry that make getting it right crucial.

i have tried this recipe, using Stevia:
http://www.healthyindulgences.net/2009/05/healthy-chocolate-cake-with-secret.html
(there are variations -- vanilla, gluten-free, etc. in case you are interested...just follow the links off the page!)