How do you make a Victoria Sandwich substituting Sweetener for Sugar ?

To make a Victoria Sandwich Cake you need equal quantities of Fat , Sugar , Eggs & Flour BUT if you substitute the Sugar for Sweetener how do you make it , what do you add to the sweetener to increase its quantity as sweetener on its own would be too sickly , what do you put with the sweetener to get the weight of the mixture right without altering its taste or consistency ?