In a saucepan, combine the sugar, sugar substitute, cocoa and cornstarch. Add water and stir until smooth. Bring to a boil; cook and stir for 1 minute or until thickened. Remove from the heat; stir in butter and vanilla until smooth. Spread over cupcakes or cake while frosting is still warm.
Editor's Note: Recipe makes enough to frost 12 cupcakes or the top of a 13-inch x 9-inch cake. This recipe was tested with Splenda No Calorie Sweetener.